Has he had it at a shop lately? One that would want his address and have had time to make a copy of his key?
I had a friend that took his new truck to a tint shop and left it there for the day. And another friend that left his new car at a stereo shop for the day.
In both separate cases, their cars disappeared a few days later.
Unfortunately, that happens. Someone at the shop makes a copy of the key and gets the address from the registration or service order paperwork.
Just a thought....