I've noticed this on a number of occasions: the listing of recent thread activity will show a thread title and specify the author of the most recent post to that thread, but upon checking the thread the named author has no recent posts. Then, some time later, checking the thread will show the named individual did post to the thread. (I haven't tracked times, so it's possible the posts were recorded as being posted while they weren't visible, but it's also possible that they didn't record as being posted until minutes later. Sorry about the lack of info.)
Is this a known bug?