A post is anything on the board. e.g. I post something today on one of the threads, the whole thread will be regarded as less then 5 days old, regardless of when it was actually created and the rabbit can then be hidden at the top of the post.
Another thing you might have noticed (or, not), an user itself is a post, so the rabbit can be hidden on the users profile, if the user has been online during the last 5 days.