Many years ago, I asked for written permission (or the laws against it, if they existed) to detect on City property in the City of Jamestown, NY, along with a definition of what was City property. In particular, I was trying to get something in writing that would allow me to detect the "avenues" between the street and the sidewalk, which the City did acknowledge was their property. The problem I had was that homeowners would insist that it was their property and a couple called the Police. The Police were nice about it and just asked that I stay away from those houses.
In the end, I did get a nice letter (which I still have) that stated that the parks and "avenues" are City property and that there are no laws or rules against metal detecting. However, it also pointed out that there is a 1941 law on the books that states that "no citizen may disturb the flowers, trees, or grass on any City owned property". I expect that many municipalities have similar laws on the books that could be used against metal detecting (or retrieval of the items).
Scott