Update: I just had this happen again, but this time in the middle of a room, and not a corridor. The monster was nonhostile at first, but after I received a message about its missing me, I checked and saw that it was now hostile. I wonder if it became hostile, then attacked me, or if it attacked me and then its status changed to hostile? What you said could be true, Stingray. Unfortunately, I didn't start keeping track of hostile/nonhostile status of the buggy monsters until I read your post.