Were there implicit laws not referenced in the Acts 15 letter to gentile believers?

Yes there are implicit laws referenced by the requirement for Gentile Christians to "abstain from what has been sacrificed to idols, and from blood, and from what has been strangled, and from sexual immorality. If you keep yourselves from these, you will do well."

First, we need to know that Jews believed that God placed certain rules in the conscience of all human beings. These are sometimes called the Noachide commandments. It is a fair assessment to say that what we have in Acts 29 is a decision by the elders that Gentile Christians need to accept an updated version of the Noachide Covenant. Among the rules normally included are:

• Do establish laws.
• Do not curse God.
• Do not practice idolatry.
• Do not engage in illicit sexuality.
• Doo not participate in bloodshed.
• List item rob.
• Do not eat flesh from a living animal

Apparently the elders felt that some of these went without saying. But others concerned the leaders enough that they felt the need to specify.

SEXUAL IMMORALITY

The OT has many rules regarding sexual immorality. They include laws against adultery and fornication, incest, h*m*sexuality and be*t**lity. These are fairly well known to most readers.

FOOD SACRIFICED TO IDOLS

The OT Law referred to here is probably Ex. 34:13-15

You shall tear down their altars... lest you make a covenant with the inhabitants of the land, and when they play the harlot after their gods and sacrifice to their gods and one invites you, you eat of his sacrifice

REFRAIN FROM BLOOD

This is sometimes interpreted as meaning murder or blood-violence. If so the refence would be found in the 10 Commandments. But this would go without saying. A better reading is that it is a requirement to eat only fully cooked meat. In that case, it is clearly one of the commandments given to Noah. (Gen. 9:3-6)

Every moving thing that liveth shall be meat for you; even as the green herb have I given you all things. But flesh with the life thereof, which is the blood thereof, shall ye not eat.

NOTHING STRANGLED

This rule has no definite referent in the written Torah, but kosher butchery came to hold that that "the animal must be killed with one clean cut across the throat and allowed to bleed to death." This was apparently already a requirement of the Oral Torah in Jesus' day, meaning the rabbis had reached a basic consensus about it. It would represent either the rabbinical interpretation of Gen. 9 (strangled animals have not yet been drained of their blood) or possibly of Leviticus 11).

These, then, are the specific OT and rabbinical referents to the rules mentioned in Acts 29. They are best understood as a Christian version of the Noachide laws, plus one or two additions based on the emerging consensus of rabbinical thought. However, I would not agree with the OP that "the dietary laws were not required of gentiles." Some of the dietary laws were indeed required: refraining from blood and not eating strangled meat. Not eating food offered to idols falls in an area that might be considered dietary law and/or the law against idolatry.)

Until Peter's vision (Acts 10 (NKJV)), in which he is told that Christianity is intended for all mankind, "… God has shown me that I should not call any man common or unclean", Christianity was entirely Jewish, the only difference being the belief that Jesus was the promised Messiah. Christians and Jews lived, worked, worshiped, and studied together. But now Gentiles were wanting to become Christians, and there was no obvious path for them to follow.

Consider how the Gentiles that were considering converting were learning Christianity:

So when the Jews went out of the synagogue, the Gentiles begged that these words might be preached to them the next Sabbath. — Acts 13 (NKJV)

In Antioch, Paul was teaching at an existing synagogue on the Sabbath. In other places, where there was no Paul or other teacher continually available, interested Gentiles were expected to attend regular sabbath services at their local synagogue. There, they could learn the basic truths about God, creation, and the history of Israel.

During the other six days, they could study the scriptures for themselves. Until recently though, books of all kinds were rare and expensive. Individual people did not own Bibles, and there were no lending libraries; anyone wanting to read the scriptures had to go to a synagogue:

[The Bereans] were more fair-minded than those in Thessalonica, in that they received the word with all readiness, and searched the Scriptures daily to find out whether these things were so. Therefore many of them believed, and also not a few of the Greeks, prominent women as well as men. — Acts 17:11–12 (NKJV)

But, even though Jews welcome Gentiles to their services, they require that those people be seriously interested, not simply curious. Giving idol worshippers, blasphemers, thieves, etc. access to the holy documents would profane them.

Modern Judaism has a concept of universal laws that apply to all humanity, not only to Jews:

The Seven Laws of Noah (Hebrew: שבע מצוות בני נח<200e> Sheva Mitzvot B'nei Noach), … are a set of imperatives which, according to the Talmud, were given by God as a binding set of laws for the "children of Noah" – that is, all of humanity.

According to Jewish tradition, non-Jews who adhere to these laws are said to be followers of Noahidism and regarded as righteous gentiles, who are assured of a place in Olam Haba (עולם הבא<200e>, the world to come), the final reward of the righteous.

The Seven Laws of Noah include prohibitions against worshipping idols, cursing God, murder, adultery and sexual immorality, theft, eating flesh torn from a living animal, as well as the obligation to establish courts of justice. — Seven Laws of Noah - Wikipedia

The meat prohibition is derived from Genesis 9:4 (NKJV), "But you shall not eat flesh with its life, that is, its blood".

Ignoring the four laws that almost all religions would consider obvious (blasphemy, murder, theft, injustice), and which the proselytes would already be following, leaves these three:

• Worshipping idols.
• Sexual immorality.
• Eating meat that has not bled to death (e.g. strangled or diseased, or still alive (oysters)).

These prohibitions correspond almost exactly to the list defined by the Council of Jerusalem.

The controversy in Acts 15 was over exactly what potential converts to Christianity (and implicitly to much of Judaism) should be required to do before their conversion.

Some Christian leaders felt that these Gentiles should first convert to Judaism. They would have to be circumcised and follow all relevant laws before they could become Christians.

Others realized that that would be an impossible requirement: no one would want such an operation before they were even allowed to learn about the reason for it. They also knew that Christianity was based on a new covenant with God, one available to all mankind, and that conditions specific to the old covenant were meant for the descendants of Abraham, not for all of humanity:

And God said to Abraham: “As for you, you shall keep My covenant, you and your descendants after you throughout their generations. This is My covenant which you shall keep, between Me and you and your descendants after you: Every male child among you shall be circumcised; and you shall be circumcised in the flesh of your foreskins, and it shall be a sign of the covenant between Me and you. — Genesis 17:9–11 (NKJV)

The final decision was that the Christians shouldn't require any signs of sincerity beyond what the Jews already required of Gentiles: they should refrain from activities forbidden to all of Noah's descendants: temple prostitutes, meat sacrificed to idols, meat containing blood.

Notice what James concluded:

“Therefore I judge that we should not trouble those from among the Gentiles who are turning to God, but that we write to them to abstain from things polluted by idols, from sexual immorality, from things strangled, and from blood. For Moses has had throughout many generations those who preach him in every city, being read in the synagogues every Sabbath.” — Acts 15 (NKJV)

• Requiring circumcision of potential converts would be counter productive; rather than encouraging them to learn the truth it would scare them away.
• Requiring only what the Jews already expected for admission to their synagogues would be sufficient.
• Each week, the potential converts could then attend services and study the foundations of the religion.

Despite how most people misinterpret this ruling, it had nothing to do with biblical dietary laws, nor anything to do with Christianity versus Judaism.

Was there some implicit set of laws that was not referenced, but would be understood by both parties to be in effect?

Yes, effectively everything. This ruling was not about what rules Christians should or should not follow, but about what potential Gentile converts needed to do to be allowed to learn in Jewish synagogues.

The laws which they are saying do not apply to the gentiles were specifically laws which were a part of the old covenant between God and Israel. Since the gentiles are accepted through Jesus' sacrifice and the new covenant, they no longer need the sign of the old covenant.

I believe the other laws which were mentioned by Jesus or still practiced by Jewish believers would still be understood to be in effect.

The laws that were stated here are laws about ones self not pertaining to love. We know from the new testament that the thing we are commanded to do is to love.

John 13:34 NIV

A new commandment I give to you, that you love one another; as I have loved you, that you also love one another.

We also know that it is love which fulfills the law.

Romans 13:10

Love does no harm to a neighbor; therefore love is the fulfillment of the law.

In the letter that you're quoting in your question, the Elders are telling people about the laws that are not under the law of love. Thus you don't have to worry about those laws. For the law is fulfilled in love.

Matthew 22:37-40 (NIV)

37 Jesus replied: “‘Love the Lord your God with all your heart and with all your soul and with all your mind.’ 38 This is the first and greatest commandment. 39 And the second is like it: ‘Love your neighbor as yourself.’ 40 All the Law and the Prophets hang on these two commandments.”

The laws that you're asking about: stealing, murder, covetousness we are still commanded to follow because those are under the law of love.