On one of my internships there was a small handheld radio floating around the office. I changed it to some local AM jazz station, set it to the lowest possible volume (such that it was barely audible), and hid the radio inside another interns desktop. I told the other interns about it, and we agreed that whenever he asked anyone if they could hear music, that we would tell him we couldn't hear anything.

At first he seemed mildly annoyed but mostly ignored it. You couldn't always hear it depending on what song was playing, so that helped keep it hidden for a while. Fast forward one week, we came back from lunch to find that the guy had disassembled almost everything in his cubicle before finding it. He angrily held up the radio and called us all jackasses. I have a little chuckle every time I remember this!