What’s this nationwide code game I heard of?

My aunt told me about this game where people hide codes at different places around the U.S., and that for a hobby or just for the fun of it, other people find the clues. There’s no prize or anything, you just hide a clue, give a hint, and it’s like a huge scavenger hunt thing across the country.
However, I can’t find anything about it anywhere online.
Does anybody know what this is?
I’m not interested in joining in on the game or whatever, I just want to know what it is.
My aunt’s daughter said someone hid a clue on the stop sign by their house, so it interested me.