What does YOUR cat do at night? Mine hunts mice, catches them, and then either a.) eats them too fast before boking them on the living room rug, or b.) releases them to run up the curtains. There’s not much mystery to it.
However, if you have no idea what your mog’s up to, ever considered attaching a camera to it? I know you haven’t, but never mind: someone’s come up with the CatCam anyway. It’s a camera, with a controller for timed shots and a protective casing.