There's a famous US counterfeiting case that was broken during the War. A bank teller who also was a coin collector noticed a "war nickel" without the mint mark. Eventually the Secret Service investigated and discovered a counterfeiter who was making nickels! He figured nobody would ever notice they were fake because they were just nickels.
The dies were excellent and the machines he used so professional that after his conviction his equipment was installed in the US Mint.
Had he remembered the mint mark perhaps he never would have been caught.
It's amazing what you can near vending machines. That nickel more than likely kept getting rejected so it was simply tossed on the ground. I found a silver rosie on top of a vending machine at work. It probably was put in the machine numerous times and rejected so the person set it on top of the machine thinking there was an issue with that dime. Congrats on you find.