I was about to say a paper 2x2 flip, but that wouldn't make sense if the obverse was more dramatically toned than this. The only obvious answer is that you took it to a Phish concert.
It was kept in a plastic case inside a manilla envelope. Then left inside a medicine cabinet in a bathroom and for some reason the exhaust fan was broken, hence an excess of humidity. The person had a bit of a bottom issue and disliked throwing away the used preparation h tubes and sometimes the caps were not on tight, causing a tightening of the silver molecules. Then the person threw out the used tubes, (had a date coming over) which caused the molecules to expand back to normal and the baking soda in their toothpaste caused the reaction.