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An old local church has been taken over from a private resident for not bringing it to code. It is scheduled to be demolished soon. My question is if its public property now that it is owned by the township?
I have seen examples of plots of land, buildings, etc.... here in my state fit that scenario too. For example: A building, or vacant lot has been in the same family for 3 or 4 generations (since the turn of the century). But the side of town has become so blighted and derelict, that the boarded up building gets to the point where it would NEVER pay to re-open whatever businesses were there. Or the vacant lot would never pay to think you could build anything on the property. The area has basically become a skid-row. So the descendants (who now perhaps don't even live in the area anymore), become disgusted with even paying property taxes on it, nor would anyone else want to buy it. So one day, one year, they just stop paying property taxes on it. The city comes in eventually, and takes over the property for lack of tax-payment. Now it's essentially city property.
To answer your question, I do not hesitate to hunt it during that time (like if the building gets demolished, those are always the best spots, since they're in the oldest parts of town

)
Now I suppose you'll have other person's say that ... yes ... it's city property (and thus "public"), but yet however, this merely means you must go ask the city "can I metal detect there?". But I would take a different approach this: Turn it around the other way, and instead of thinking you need "express permission", instead, check to see if there's any prohibitions (specifically saying "no metal detectors") in city muni. codes. If not, then .... to me, you don't need permission for something to which there's no prohibitions. City laws, muni codes, etc.... are usually on the city website. Presto, just do a key word search there.
You can certainly go asking at city hall, if you want. But after your question gets passed back and forth between various desk-bound clerks, you may just get a "no", when ...... truth be told ...... no one may ever have cared or noticed.