Some vending machines will give you back different coins that you put in. Try this, put a state quarter into that machine and then, reject it. See if you get the same state back. If you don't then put two dimes in a few times and see if there are more silver coins. Coincrazed did this recently in Vegas. That is how some metal detectorists, get rid of their dirty clad finds.
I was told that people who own vending machines or clean out the coins are at the coin shows selling silver coins they found. Keep eyeballing the vending machines!