Pair of good quality 10 x 50 Binoculars under the passenger seat. Being that sort of person, I took them round to the previous owner. He appeared rather surprised and took them. On reflection, I now think that they were not his but a previous previous owner's. Also found a nice watch - I kept that - and it still keeps perfect time and looks well. Over the years, probably found in excess of 100 notes in coins etc under carpets. I kept them too...
An MGB GT I bought back in the 1980s had a brand new ignition key bolted to the front bulkhead. I think it had been there since the day it left the factory.
Taking delivery of new cars, have found several new parts such as side indicators and parts of spare loom in passenger foot well. Odd ash trays door handles, bonnet release handles and other small new parts under seats. I guess that when someone drops a part during assembly, with the thing on the move on the production line all the time it may be quicker to get another part out of the bin and not hunt round for the dropped one.
Probably best of all was a Montego Turbo I bought from a neighbour about six years ago. This had an expensive brand new motorcycle silencer (about two hundred quids worth), a new trailer spare wheel and numerous useful tools such as brake self-bleed kits and other stuff ... a real boot full of useful stuff. I did not check the boot contents for about a month. When I took most of the stuff back he said something like "I wondered where I'd put some of that stuff" .... some people obviously can't be bothered to empty their cars when it changes hands.
Funny old game people and cars.