Back when Pepsi & Coke came in returnable bottles, a lady come into the store where I worked with a 12oz., unopened bottle of Pepsi from the 6-pack she had just bought and held it up to the light. Yes, there was a mouse on the bottom. One can only assume it got in when the bottle was used before, and was not discovered during the automated bottle-washing process and inspection.

None of us were clever enough to keep the bottle and turn it into a gold mine.

(we also found stuff like rusty bolts in bottles occasionally)