Oh, it's a high visibility spot, right next to a major roadway. Just to clarify, the location is in a National Recreation Area and according to Wikipedia, it's under the jurisdiction of the National Forestry Service.
I think you mean U.S. Forest Service? There is no National Forestry Service in the U.S.
Several people a year are fined or arrested for possessing a working metal detector in a National Recreation Area. Keep the detector locked in your trunk with no batteries and you should be alright in the NRAs and National Parks/Seashores.