A CO detector is really only needed in a car that isn’t well maintained. If the exhaust system is complete and leak free, there is no danger. Even less danger with a modern fuel injected car as the exhaust has very little CO in the exhaust stream.
The issue you may have seen recently with Ford Explorer police vehicles and CO is due to a cracked exhaust manifold allowing gas to leak under-hood and into the fresh air intake when the SUV is idling and stationary. Like at a stake-out, speed trap or donut shop parking lot (forgive me for the donut crack, I couldn’t help myself).
You shouldn’t leave the car running in the garage with the door down so the CO detector isn’t needed there either. Mount one in your bedroom if you have a gas furnace or living room with a gas fireplace.