We would suggest the Rear Window Camera module, which adheres to the rear window. You can read about our test with that particular module in the Nextbase 422GW review (kit.honestjohn.co.uk/reviews/review-nextbase-422gw/), and I've attached the 522GW review below. It's unlikely that the rear camera will fall off because it's attached via a very stong 3M adhesive mount. Just make sure it's in the correct position to film the road before you set off. In our experience, it tends to be pointed lower than you'd assume. We've got a lot of footage of the sky in the past...

Nextbase 522GW review: kit.honestjohn.co.uk/reviews/review-nextbase-522gw.../

As for leaving them in the car, we tend to leave our dash cams clearly visible all the time. This is dependent on whether you park on a driveway or on street and whether theft from cars is a big problem where you park. If you're worried, we'd recommend taking it off the window for peace of mind. The Nextbase Series 2 dash cameras are easy to detach because they're powered via the mount and the camera itself can easily detach and reattach to the magnetic mount, however, we've never had an issue just leaving them on show.