A few times recently I've noticed a question coming up to the top of the list in the forum, marked as having had activity by a user, but when going into the question not being able to see anything.
A recent example (screen shots at 12:06 UTC):
but in the question:
I wonder if the mechanism is picking up abandoned edits or posts?
asked 30 Mar '12, 08:10
I agree that this is a source that makes me sometimes puzzled as well - though not in this case, for obvious reasons:)
But it's behaviour by design IMHO: This will happen when a user just converts a comment into an answer or vice versa, which is not visible in the topic afterwards - however, that user's recent acticity view will give more clues:
I've seen this as well. There are a number of operations that do not get recorded with the user's name, notably changing a comment to an answer or changing an answer to a comment. The other reason that I know of is if a user adds a comment or answer and then deletes it - in this case the page has no remaining signs of the operation yet the summary page will continue to show that there was activity on the page.
Duh: now I see the entry in the log! As Volker mentioned, he changed an answer to a comment.
And another reason for this to happen is if a user edits a comment since the page does not show information about edits on a comment.