During the bad old days of labor disputes and union misbehavior, auto workers would "accidentally" lose tools in car doors or parts behind the dashboard. Those days are over. That means workers now just legitimately lose stuff. Here's some examples.
Despite the widespread automation of the auto industry and drastic reductions in factory workforce, there are still a lot of people involved with screwing a car together, especially in the final assembly plants. No matter how agile and dexterous a robot might be, sometimes it just makes more sense to have a human wiggling something in place and bolting it down. It's in that process though that sometimes stuff just disappears.
Occasionally things lost will be bad and have legitimate consequences, say dropping a screw down an air conditioning duct. These things happen and when they do workers are trained to signal their team leader who decides to either stop the line and fix the problem or send the car forward and mark up a detailed description of the repair work needed at the end. More often then not though when something gets lost it's harmless, like in the case of VWVortex member "2 doors" who showed off some of the goods he found while working on his cars.
After rummaging around in a Mazda6 to fix a broken cable he noticed a lump under the carpet under the seat, curiosity got the best of him and he discovered the lump was caused by a stray antenna mount which had somehow grown legs and walked into the car. More amusing was a pilot nut driver he found in a VW GTI that had somehow been overlooked on final inspection and caused yet another lump in the carpet.
Modern assembly factories are practically white collar places, the levels of professionalism and dedication to quality is usually shocking to folks whose opinions are based on stereotypes of plants as dens of cretins. As a result, this kind of thing is exceedingly rare anymore, but when you make thousands of cars sometimes a few extra pieces make it out the door, or maybe your car has an extra brass hammer under the trunk lining.
Have you ever found any hidden treasures in your car?