According to the answers given to a recent question of mine, the Penitential Act in a Mass, which involves an absolution from the Priest, does allows for the possibility of forgiveness of venial sins. Yet, such rite is not efficacious when it comes to mortal sins, which require the Sacrament of Penance.

Also, notice that the Sacrament of Baptism also involves a forgiveness of sins (including the original sin). According to the Catechism of the Catholic Church:

1263 By Baptism all sins are forgiven, original sin and all personal sins, as well as all punishment for sin.66 In those who have been reborn nothing remains that would impede their entry into the Kingdom of God, neither Adam's sin, nor personal sin, nor the consequences of sin, the gravest of which is separation from God.

If you're just talking outside of the normal priest-to-penitent sort of confession, there is yet another proviso in Canon law besides the provisos already mentioned called "General Absolution":

Can. 961 §1. Absolution cannot be imparted in a general manner to many penitents at once without previous individual confession unless:

1/ danger of death is imminent and there is insufficient time for the priest or priests to hear the confessions of the individual penitents;

2/ there is grave necessity, that is, when in view of the number of penitents, there are not enough confessors available to hear the confessions of individuals properly within a suitable period of time in such a way that the penitents are forced to be deprived for a long while of sacramental grace or holy communion through no fault of their own. Sufficient necessity is not considered to exist when confessors cannot be present due only to the large number of penitents such as can occur on some great feast or pilgrimage.

Canon 961

That's the case that covers mega-volcanoes, cataclysmic meteor strikes, and non-trivial alien invasions, etc...

But, the faithful are still instructed to go to confession (especially if you have grave sins) as soon as possible.

It would be an interesting follow-up question to ask when this actually has been granted in the history of the Church, might even be worth making a wiki page out of.

But here are three such events from American history (via wikipedia)

• Fr. William Corby in the Battle of Gettysburg in 1863
• Three Mile Island nuclear accident
• New York firefighters heading into the World Trace Center on 9/11

Can a person be forgiven outside of confession?

It seems that no one has taken up the point of martyrdom yet. It should be added to the list of previous answers.

True martyrdom requires three conditions: (1) that the victim actually die, (2) that he or she dies in witness of faith in Christ which is directly expressed in words, or implicitly in acts done or sins refused because of faith, and (3) that the victim accepts death voluntarily. They are not martyrs who do not actually die, or die from disease, for the sake of merely natural truths, or heresy, or for their country in war, or through suicide, etc. - True and False Martyrdom

In the Early Church only martyrs were honored as saints. In 197 AD, Tertullian wrote: “The blood of martyrs is the seed of the Church.”

The Church has always held that martyrdom is equivalent to baptism for those not yet baptized. Baptism of Blood is the name given to this. It is Catholic doctrine that the Baptism of Blood blots out Original Sin, and all actual sin, together with the punishment due to it. This is evident from the words of Christ: He has absolutely promised salvation to those who give their lives for the Gospel: "he who loses his life for my sake will find it"; and again He says, "So everyone who acknowledges me before men, I also will acknowledge before my Father who is in heaven". Moreover, from the Tradition of the Church: the Church honorrs as martyrs in heaven several who were never baptized: the Holy Innocents massacred by Herod; St Emerentiana (c. 304); one of the 22 Ugandan martyrs, St Mukasa Kiriwawanvu, who was still a catechumen. St Augustine says, "it would be an affront to pray for a martyr: we should rather commend ourselves to his prayers." (Sermon 159) - True and False Martyrdom

Here is what the Catechism of the Catholic Church has to say on this:

1434 The interior penance of the Christian can be expressed in many and various ways. Scripture and the Fathers insist above all on three forms, fasting, prayer, and almsgiving, which express conversion in relation to oneself, to God, and to others. Alongside the radical purification brought about by Baptism or martyrdom they cite as means of obtaining forgiveness of sins: effort at reconciliation with one's neighbor, tears of repentance, concern for the salvation of one's neighbor, the intercession of the saints, and the practice of charity "which covers a multitude of sins."

CCC states that unbelievers can achieve eternal salvation- I would assume forgiveness is necessary for this, though I don't understand how this is reconciled with the biblical account.

CCC 847

Those who, through no fault of their own, do not know the Gospel of Christ or his Church, but who nevertheless seek God with a sincere heart, and, moved by grace, try in their actions to do his will as they know it through the dictates of their conscience - those too may achieve eternal salvation.

(CCC Online)

According to the Pope:

It must first be kept in mind that every quest of the human spirit for truth and goodness, and in the last analysis for God, is inspired by the Holy Spirit. The various religions arose precisely from this primordial human openness to God. At their origins we often find founders who, with the help of God’s Spirit, achieved a deeper religious experience. Handed on to others, this experience took form in the doctrines, rites and precepts of the various religions.

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It will be in the sincere practice of what is good in their own religious traditions and by following the dictates of their own conscience that the members of other religions respond positively to God’s invitation and receive salvation in Jesus Christ, even while they do not recognize or acknowledge him as their Saviour

(Full Text)

These instances would necessarily be outside of the sacrament of confession.