I discovered what is living in my apartment

I heard something rustling around again last night and went to investigate.

I followed the noise to a white plastic bag that, among other things, contained some treats for my parrot, so The Thing was apparently having dinner. I snatched up the bag, but the creature was quick and I saw a dark shape dart out of the bag and behind a board that was propped against the wall. I pulled the board away, and there was no mere mouse behind it, but a giant fucking RAT. It dove under my work table, into a wadded-up plastic tarp. I stood there with my jaw dropped for a moment, and then decided that I didn't really feel like dealing with a giant fucking RAT at 3am, so I went back to bed. I left the light on in hopes that it would frighten it into remaining in the tarp while I got some sleep, but NOOOOOooooo, it kept me up ALL night, scratching at things and running around. I gathered together a pile of cheap Home Depot paint brushes, and used them as projectiles every time I saw it run out across the floor, which did nothing to alleviate the situation, but it was somewhat cathartic.

I think it's only been in my place for the past couple days, but I'm wondering how the hell it got in? I have something of a hell-mouth in my back laboratory slop-sink room, and while I typically keep the door to that room closed, I did leave it open when I ran upstairs to throw some laundry in the drier on Sunday night, so maybe it ran in then. Who knows.

I called my building management and they're sending Pest Control over to deal with the situation. Let's hope this little unpleasantness will be over by the time I return home this evening.