Assuming they're sleeping in the car with the AC running, my car never seems to let me keep it on recirculation for more than like 10 minutes. I noticed this while I was driving through a bunch of forest fire smoke I was trying to keep out, and I had to keep turning it back on.

Maybe that's just a safety feature my car has, but not all cars? Or am I misunderstanding and it's an issue with exhaust fumes? That would definitely be an issue in a garage.