I've looked into these little 'hidden' camera's before. I took a look online at the sort of devices available including looking to Maplins.
There is a reasonable availability and range of products. Of course there's a range of prices too.
Mostly they are activated by movement within a certain range, i remember some could be set to constant recording though.
Most of these recorded onto SD card and some could record a very good length of time without any looping.
Might be worth some research as a lot of the cameras were quite cheap, very small, disguised (hidden camera) and easily installed.
Food for thought