Guess I'm leaning toward calling the cops. Not sure which ones would be the right ones where you are.
It reminds me of when one of the gals where I worked got a visit from the local cops about 'her' car they found at the bottom of cliff. (probably an old quarry) Took a while (several weeks) to straighten out 'cause the purchaser had not reregistered it. Otherwise the fines were going to be on her.
That was NJ.. I'm sure some places it can be more complicated.