Spy recorder device?!?

my partner's stepdad was thinking about putting a tiny tiny camera on the windshild of his girlfriend's car to see if she was cheating. he said the one he was looking at was as big as a stick of gum. there are recording devices, you would need to have one that turned on when there was sound and turned back off when there wasn't so you wouldn't end up with it empty. i would ask at radioshack.