To reproduce, create a very stealthy character and spawn a fire vortex. The vortex flits around as if it is awake, but it has the 'zzz' mark on its tile and is described as "resting" on x-ving it. I'm not clear on whether there is any difference in its behavior when asleep vs awake.
On reflection I'm not sure this is a bug instead of a special mechanic. (Although it's a pretty minor monster to get its own unique mechanic.) If this is intended I suggest something be added to the monster description, since it's unusual behavior for monsters in this game.
Possibly this is related to monsters "sleepwalking" when both confused and asleep and/or to monsters continuing to wander a little after falling asleep (5099, 11987).