[SPOILER] A second smoke monster...

[Funman]

Actually, all of the monsters are "smoke monsters," though not all of them are hallucinations (hallucinations are the ones which fade to smoke if you get near them). This is true because all of the monsters eventually disappear, you just may not be around to see it. They're not constantly patrolling, they're usually on a set path which they only leave if they hear or see you. If not, they continue on and eventually fade away...