I'm actually surprised this doesn't happen A LOT more, and it probably does. Cameras these days are so tiny, there's little chance in finding one unless maybe you have a wifi sniffer, and then that only works if it's not just doing onboard storage.
I know there are detectors that let you sweep for a digital lens, but I don't know how much they cost. Probably not cheap though. There might be a cheap way to MacGuyver one.
Edit: I was curious, and a quick search showed this homemade method:
Before you spend any money, we found that the DIY version of reflective detection worked just about as well:
• Grab a paper towel roll and a flashlight and hold the roll up to one eye and close the other.
• In a darkened room, turn on the flashlight and place it at eye level next to the tube and slowly scan the room for the telltale white reflection.
We found every camera in our test room with this method, so give it a try at home yourself.