I made one of those evil things in college. It was built on a 555.
Actually, to be REALLY evil, it MUST have a light sensor. Then you can place it when they turn their back and it won't turn on right then. It'll turn on when they try to go to bed. Then they'll turn on the light to go find it, maybe unplug some appliances, don't hear anything, go back to bed... and it chirps again.
Man, annoyance that begins right as you try to go to sleep is FAR more annoying than right when you come back from class. Very low tolerance for this.
I had the light sensor designed for total darkness and it stayed off when dropped behind his couch. It worked as designed and he ended up unplugging everything in his room, took the batteries out of everything, man he was pissed. He went through several cycles of looking for it (lights on), waiting in the middle of the room tensely to figure out the direction, heard nothing, nothing, nothing... 5 min later goes to bed... BLEET. I don't think he ever figured out the part about it being connected to the lights being off.
I was afraid that thing was going to end up in the wrong hands. I had reason to be afraid. Testing showed there was no way at all to locate that high frequency sound that bounced all over the room. Very difficult even if we left the speaker on continuously. With the mess in our room I knew we were screwed if they dropped it in on us.
*I* knew to sleep with the light on though to keep it quiet.