I've been using a cheap PDA as an annoyance device. This model has an alarm function that turns off after it cycles through for about 20 seconds. It will not resound for another 24 hours. The alarm function stays "live" even if the display is turned off and that extends the button battery life considerably.
It drove my father batshit crazy. I placed it on the top of their china cabinet in the dining room. I programmed the time to correlate to a few minutes into Wheel of Fortune.
It took them about three days, but they found it.
However, when planted at a girlfriend's house, she did nothing. She was aware of the sound, but thought it was a glitch in her security alarm panel or microwave oven panel and didn't look for the source.
You can buy practical joke sound devices designed especially to do this, but make sure they shut off after a few seconds. Some just keep beeping at random intervals until turned off or found and that's useless.
I received this one FREE from a magazine promotion, indicating how little they are worth. Many such devices were dumped on the market when cell phones began to evolve, making dedicated organizers pretty much redundant. This model even has a stylus stored in a back slot. Sometimes found on ebay as "electronic organizer" for a couple of bucks, though some dealers believe it's worth 15 or more, which is insane.
Next: to hide it in a friend's car, up under the dashboard.