Unknown Leather Seat Spots?

Possibly mold. Try spraying a spot with 3% hydrogen peroxide to see if there is a bubbling reaction. If so, you might let it soak and see if that might be an effective remover. I cleaned mold off of a down jacket with the stuff.

 

I was thinking mold too. Like the car sat for a protracted period, all sealed up.

 

...hmm mold OK that might be a good diagnosis.
Sometimes used cars are subject to ozone treatment to remove odors, and sometimes excessive ozone treatment can discolor plastics....that's a long shot.

 

Or it was flooded...

 

Just try a small area to test it first. I suspect hydrogen peroxide won't hurt the seat, but it if is mold, it's already damaged if you don't fix it.

It is just that one passenger-side front seat? Maybe the previous owner had a seat cover on it that got wet and stayed wet underneath unnoticed for weeks. If it's all the seats, I'd worry about flood damage and look for other signs.

 

Also check Google for how to handle mold stains.
I spent a little time looking, they seemed to be saying dilute vinegar.