It is amazing the stuff you find in a second-hand car when you start cleaning it out thoroughly. I have twice contacted PO's to ask them if they want their wife's watch back, or something similar, like house keys that i found once. These were past POs, not the guy i bought it off. they were amazed and grateful. One time I called up a mechanics number under the bonnet and he gave my number to the PO who then called me up and we got on great and I found out the info I wanted. I even sent the bloke $500 cos he got ripped off after it broke down and he sold it to the mechanic interstate where it broke down. He was happy to then send me more spare keys and the vehicle's history.
No harm if you approach respectfully. If they then treat you badly, the sin is on them IMHO.
But funny yeah, with new technology and ppl leaving their address in the satnav LOL - they cannot complain at all if you drive up and ask for a beer!