I’d also look for a lockbox in the house. Check closets and basement. Do you have his keys? Can you account for every key on the ring? One of them might be a clue. I doubt he would have kept them inside the cars, that makes life too easy for car-jackers.
I have about 25 keys that I have no idea where they go. He was a driver for the Amish. He had wrecked about 7 vans and they were junked right away. Usually before I was told about it. He would have had to clean them out, not me.
He had a two car garage and a 4 car garage that was full. It has taken me about 4 months to get through all this stuff.