Quoted from djreddog:For the office, a few years ago I bought a ThinkGeek Eviltron and stuck it up underneath the bathroom sink. I set it to the “hey can you hear me” setting. It took 3 months before someone found it. Everyday there were stories about people supposedly hearing things in the bathroom. It was epic.
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For home use, I told my fiancé for my birthday I could really use some wireless extension cords. She went on the hunt and was relentless in trying to purchase them. Every time she added them to the cart she was getting errors. Finally she read the description and she laughed about it once she figured it out. She felt dumb.
https://www.thinkgeek.com/stuff/41/wec.shtml
Oh man, someone put something like this under a desk at work. Eventually the facilities team put that desk in storage and it ended up getting redeployed for someone on the team I'm on, it took us months to figure out what the hell was making noises randomly. We thought it was coming from the wall and they were preparing to cut it open to see wtf was doing it.