My city and county have their ordinances online for anybody to read. Public employees just simply can't make up "rules"...they are not dictators. Ordinances and laws have to be proposed, discussed, and passed, so the people in charge have accountability to the people that pay their salaries...and one of those people is you.
I'd go peruse the county website and find the ordinances/laws/etc. and search them for "digging", "metal detector", etc. And then--depending on what you find--either go do what you enjoy or not.
For an example, there is an almost 60-acre soccer complex and equestrian center not far from my house. There is 6' chain link fence around the entire thing, with several walk-through gates. Beside every entrance, there are very large white signs with red lettering that states that the place is managed by the Futbol Club of NW Florida and that all activities must be approved by the club. There are restrictions about all kinds of random !!!!.
I can clearly see on the county GIS map that my county owns the land. So, 3-4 months ago, I was a bit cautious when I first started beeping the place. Nope, I never asked permission...didn't even think about it, because my tax dollars can't be used on valuable acreage to benefit some private organization. Scrooge that. So I get up in the morning at 4 am on the weekdays and go beep until I have to get home and work. I beep on Sunday and my days off as long as I want. Nobody has ever said a thing to me, and I dare them to. And I've been in the middle of the soccer fields when there were hundreds of people there (on other soccer fields, obviously).
Not far from that complex, there is a baseball complex that has 15 or so baseball fields. Same thing, owned by the county, so I go whenever I want. I don't make a mess and I take my trash. In the 7 months or so that I've been beeping, just out of those two complexes, I've founds thousands of coins, many pieces of jewelry (rings, necklaces, bracelets, pendants, charms), and some cool stuff. Neither place is older than 20-25 years.
My point is that asking some territorial jackwagon questions about his fiefdom is a guaranteed fail. Find the laws and/or ordinances. Read them, learn them, be able to recite them (or print out the pertinent sections and carry them). As long as you aren't littering or leaving plugs laying around or making some other kind of mess, you are using the public property for recreation, just like everybody else. I mean, don't be a sphincter about defending yourself, because I've been arrested more than once in my life for emphatically defending my rights...which taught me that you can be arrested for anything or nothing. You may be correct in the law, which will set you free...later. Jail sucks.
Go have fun and happy hunting. Hope you find something awesome!
Oh, BTW. That park director is full of bullsqueeze. So, if somebody leaves their purse/wallet/bike/car/whatever on parks property, all of that stuff is then the property of the county? Yeah, no. Telling you, petty apparatchiks are idiots. Ignore that nonsense and go find the law and read it.