These might already exist and I just haven't run into them yet, but if they don't, how about smoke grenades? You pull the pin, drop the nade or throw it, and the target room gets filled with a thick smoke for 30 seconds or a minute or whatever seems balanced. Smoke makes it incredibly hard to see people come and go, and maybe you only see a percentage of the things in the room during that time based on your perception?
Possibly makes you cough if you stay in the room too long?
