Agreed, are we looking to see if your house cleaner is stealing, partner cheating, door man coming up and stealing.....what is it? Context can help to see what avenue you should take
Well we need to know more about what you're trying to capture, duration of the anticipated behavior, where specifically you expect it to occur, type of room and what's in it, how big/small it needs to be hidden. Legally, should be good in your own home unless you're peddling toilet cam vids.
Look on Amazon. There are thousands of choices, from alarm clocks to USB charging bricks. I use various shapes and sizes to monitor my drones, especially those slackers in sector 7g.