Well, I've thrown some of those screws out and THEN discovered where they fell out of. And I bet if I hadn't thrown them out then I would have never discovered their origin.
I mean... if your room have a lot of stuff then it might take a while to find the source, if you aren't being pranked. But random screws on your desk will never be as horrifying as random maggots on your desk.
I like to keep a few mixed in with my pocket sand. Increased effectiveness if you gotta throw sand plus you can randomly drop them on people's desks and whatnot.
I got a jar on my desk. Its filled with random bolts and nut from various projects. If I just need a one of I'll search the jar instead of reaching for the sorted bags.
I keep the good looking screws if I'm pulling something apart as well, filled a whole bucket of old screws, bolts, and nuts and it comes in handy often!