One time I was paid to ferry a C172 between a small airport outside Atlanta and Frederick, MD. I arrived at the FBO in Atlanta to be told that the owner called trying to reach me, contrary to promises the keys aren't under the passenger side floor mat and the aircraft doors aren't unlocked--the keys are in MD.
Every shop has their version of the abandoned key coffee can, shoebox, whatever--the container into which the shop tosses abandoned keys belonging to aircraft they no longer service.
That cold and windy morning I wandered into the local shop and asked to borrow their abandoned key collection, which, at this shop was a large shoebox full of keys. I wandered outside, up ended the contents of the shoebox onto the tarmac, and started trying keys one by one on the aircraft's door lock, dropping each unsuccessful key into the empty shoebox one at a time.
The third key opened the door.
