Not that I'm military or anything, but I have trained with them, and I've never heard of marking a room as not cleared. That could be extremely dangerous and would go against most departments protocols and common sense (and I'm not trying to question you, just show how it could be dangerious if we're talking about clearing a building from hostiles, unless it's something like checking a warehouse for vandalization of something).
But anyway, we've used the cheap dollar store glowsticks (like a 2 or 3 pack for $1) before to mark a room as cleared, as well as things like chalk on the doorjam, or IR glowsticks if you're using NVGs. They usually last for 8ish hours, so they last for the entire training session, or in your case a full shift. They're idiot proof (no batteries to worry about, no throwing them down quickly breaking the light), and you can put them back in your pocket and they won't show any light. I don't have any recommendations about an actual flashlight though (sorry).