I wonder how often retired couples, who may habitually travel together, have completed trips without the driver ever realising they had forgotten their own car key ?
We habitually put the 'drivers' key in a centre cubby hole while driving. The driver has to remember to remove it again and there may be a latent fear of the car locking itself with the key inside, but I think this is unfounded . It does reinforce the need for regularly checking we are still physically in possession of necessary keys.
We need to do this anyway before leaving the house. The front door can lock itself on the latch so we always go through a routine of physically checking at least one of us is in possession of the necessary house key and designated car key.
Actually I have a house key on the car key ring. I know this is not advisable. If a thief found, or stole , your car keys they could not only steal your car but in theory use the satnav to drive to your home and burgle your house. As a precaution I have set 'home' to another address a few hundred metres away. (I can find my own way from there

) And take care not to leave documents with my address in the car.