No shit. I'd let em know I knew about the crop somehow though. Like leave em a note of some sorts, congratulating them on their achievement. Also though, scolding them for allowing you to find them because if one untrained punk can find em the pigs can find em. Especially with the whole walking away from the scene thing. Getting busted could ruin their teenage years when it comes to growing. Not good.


So in summary:
Don't steal. Stealing's bad.
Let them know you found them so they think twice and hide them better next year somehow.
Maybe even leave a number if you're brave. They might smoke you out one day.