My pranks are more of the "harmless head game" pranks.
I had this "find your missing keys" gizmo that was of very poor quality. I stopped using it because it would spontaneously go off randomly every few hours. When it went off it would make a beeping noise... only it used a high frequency which ironically made it very difficult to tell where the sound was coming from.
There was a guy at work who liked to pull pranks every now and then, so I thought I would play with him a bit.... for several weeks.
I simply hid the little beeping "find your keys" gizmo in an old printer that didn't work anymore. I made sure I told everyone except the target of the prank.
It was great... as expected the gizmo would start beeping for a few seconds at random times throughout the day. The guy went NUTS trying to find the source of the mystery sound! lmao! We were all technical people responsible for the computer and printer equipment, so he knew how to take things apart. When I noticed he was becoming too interested in the printer where I hid the gizmo, I moved the gizmo up above one of the ceiling tiles.
After a week or so he started to figure out it was coming from the ceiling and started digging around up there... so I moved the gizmo under a table.
This went on for about two weeks... he would get close and I would just move the thing somewhere else. To make it even better, since everyone knew what was going on they would tell him: "Yeah, I hear it too... try looking in the [Insert random place in the office]."
Oh yeah, he was pissed when he finally figured out what was going on!