Just new here. Collection of Nat. Park Quarters, thought it would be fun for a hobby. All are marked with a P or a d so far. Today I see one from Kansas that has a s on it. What is this about? Thanks ahead of time for any help.
Assuming you found the S minted quarter in circulation it's not unheard of finding a proof coin in the wild. Possibly some unknowledgeable sticky fingers found their way into someone's collection and spent it for some candy.
Not necessarily a proof coin. The S mint ATB Quarters were also sold in a circulating finish by the roll and bag directly from the mint from 2012 - 2021. A number of these turn up as someone has ordered and searched for error or high grade and put them in circulation.