For one thing, I back into the garage so if there is a big dark thing where it should be open space, I'm not going forward.
I imagine people have backed into closed doors but is seems hard to believe anyone would put a car in reverse and move without looking first but I'm sure some would.
Getting back to the CO problem, I know people that always leave their keys in the car. If you do that in the garage it would be easier to not turn the car off as there would be no beeping on exit. While it may be handy, it would be poor practice even if the garage is locked. If you go to the store, you would still want to take the key with you when you stop. I keep my key in my pocket when driving and on a shelf in the house when home. That said, once when I used valet parking the fob was in the cup holder and I did forget and leave it there when we got home. Lesson learned.