My room mates are both on vacation and I just came home to this.

One of them has a daughter with keys, but I messaged her this morning and she's back in the city where she studies. I messaged her because this morning I couldn't find my keys ANYWHERE. I took my whole room apart, even moved the mattress. Also noticed that this morning a door to a closet was open, when my boyfriend and I never go in there. I figured it could've been a cat, but with this glass around now, safe to say I'm a bit spooked. Any way to tell if there's a carbon monoxide leak quickly? (lol)