I would go online to research the current rules. Also, I would probably go back on his days off.
One thing I would not do is think of him as a "Joke" because "a groundskeeper at a park is lower than a school custodian". Maybe he takes a lot of pride in his work. ? Would YOU like to be out there in the pouring rain and the burning heat trying to keep the field looking good when those da** kids just run all over and tear it up and then you have to repaint over all the graffiti and empty the garbage and pick up the dog poop and scrub the toilets for $12 bucks an hour
I might've tried to talk to him about his fear that I would ruin all his hard work, but it sounds like he was already pretty hot and it can be difficult to keep your head when someone is being confrontational. So maybe I would give him some time to cool off, and then try talking to him again, without my detector in hand. IF it was that important to me. Luckily, I live in an area with lots of parks, so I also have the choice of just moving on to the next one.
While I'm still in rant mode, let me just say that I've known some excellent custodians and groundskeepers, and have even learned a few things from some of them. While I'll agree that when someone starts talking to me about "their" field, when it obviously isn't, that's usually a sign of a small mind or a big ego (or both!), I just can't get my mind around this idea that because someone is "just" a groundskeeper that somehow makes them "lower" than me.
OK, I'm done ranting. (Can you tell I work in "Public Service"? he he he)
Back to the original question... I would've left when asked to leave... but then... I'm a big chicken!!
Plus, I'd also be figuring out my next move.
HH - KIlika