Does the Seal of Confession include saying whether or not a person confessed to them?

Geremia has a good answer on the Church's teaching. So why might it be that revealing whether someone went to confession or not would be a bad thing? Imagine if you will...

Mr and Mrs Smith have been having difficulties in their marriage, and (being pretty devout Catholics) have spoken to their pastor about them. He's given them counseling in the matter.

Mrs Smith has for a while suspected that her husband is having an affair. He was "working late at the office" last night, and went out for half an hour early this morning. Mrs Smith knows that the parish church holds confessions after the morning Mass, and finds a "How To Make A Good Confession" pamphlet in her husband's jacket. She goes immediately to the pastor. She explains that she suspects that her husband committed adultery last night, and out of guilt went to confess this morning. She can't think of anything else he'd be confessing. In her mind, knowing that he went to confession is tantamount to knowing that he's having an affair. The pastor knows that the wife's suspicions are true - he heard Mr Smith confess that very thing face to face. But he can't reveal that he heard the confession without potentially harming Mr Smith:

A confessor is prohibited completely from using knowledge acquired from confession to the detriment of the penitent (Canon 984)

The fact that Mr Smith went to Confession is surely "knowledge acquired from confession", and would be used to his detriment. Thus the priest may not reveal this information, and must say simply, "I'm sorry, I can't discuss confessions" or something similar.

No, a priest cannot even reveal that because it would betray the penitent.

Can. 983 §1. The sacramental seal is inviolable; therefore it is absolutely forbidden for a confessor to betray in any way a penitent in words or in any manner [e.g., by signs] and for any reason.

Can. 984 §1. A confessor is prohibited completely from using knowledge acquired from confession to the detriment of the penitent even when any danger of revelation is excluded.
§2. A person who has been placed in authority cannot use in any manner for external governance the knowledge about sins which he has received in confession at any time.

Technically, a priest can reveal good things about a penitent he has heard in confession. This is what St. Catherine of Siena's confessor Bl. Raymond of Capua was able to do in his biography of St. Catherine; ibid. pt. 1, ch. 2 fn 1:

“The virtues and graces of the penitent are not matters for the seal [of Confession], provided they are not manifested in order more clearly to declare the sins themselves, e.g., the gravity of ingratitude towards God*.” (Emphasis in original). (Jone, Moral Theology, no. 618, Imprimatur 1961; TAN reprint 1993).—Publisher, 2003.

^{* ∵ the more graces one has, the greater his ingratitude could be}

It depends on the situation.

example 1

Mr and Mrs Smith notice their Son has an increased interest in faith, they may be happy and ask Fr Mike...

We're very happy Tom is going to Mass and getting involved in his parish, we hope he's also going to confession aswell, is he?

There would be nothing wrong with Fr Mike revealing that Tom has been attending the weekly confession services.

He would not be betraying Tom, just like Geremia's example of St Catherine of Siena it only adds to the positive image of the person.

example 2

One day Fr Mike heard Tom's confession and the worst thing revealed was that he failed to wash the dishes at home.

A few weeks later Fr Tom gets an informal knock on the door from the police, they're asking if Tom revealed a violent murder.

So Fr Mike tells them...

I can't reveal what Tom said, but he definitely was with me at that time in a confession service.

Therefore freeing Tom from the suspicion of the police.

In both of these examples, Fr Mike has not betrayed the penitent in any way, either contributed to their positive image or saved them from unfair punishment. Not in any way in detriment to Tom.

So yes, a preist can reveal that someone has attended a confession service.