Sounds like it could be a potentially dangerous automation to rely on, proceed with caution:
- False positives could result in a dangerous CO build up but could be mitigated by a CO sensor as a failsafe as you said.
- False negatives could result in your garage door opening in the middle of the night or when no one is home.
In addition to what was said before me, you could use a RPi Zero in the USB port which is only powered on when the car is on and use the monitor (or presence) script, here: [monitor] Reliable, Multi-User, Distributed Bluetooth Occupancy/Presence Detection