It appears that if you set yourself up to get email notifications of posts to a thread, and then set your email client to automatically reply to email with a message that you're out of the office, these automated replies are sent to the thread. For example, the replies from gschrepfer in this thread, and I've seen the same in at least one other thread.
Mind you, I'd expect to have seen a lot more of this if it was a problem for ALL users. Perhaps there's something configured wrong on his account or some such? Anyway, it seems like something that would be good to prevent if there's any way to do so.