The "how we roll " allusion by mud-puppy, is along these lines:
When someone asks about whether a place OF ANY SORT is "legal to detect" ? Is when you enter in to 50 shades of definitions and "gotchas". In other words: NO PLACE is "legal to detect". Depending on how one words the question, and who you ask, you can ALWAYS find a "no". And even have some ancillary language pointed out to you that could be morphed to apply.
Yup, every single sandbox, no matter how innocuous, you *could* find some law or rule that .... depending on which pencil pusher or archie you asked *could* say "that's illegal".
Also, even the term "abandoned" is full of contingencies & debate. Like, well, do you simply mean currently un-inhabited ? (perhaps the owner lives down the street and is hot & bothered about everything). Or did you mean abandoned and it's been taken over by the city or county with eminent domain for back-taxes ? If so, then the debate ensues of "does that mean you have to go grovel at city or county for 'permission', lest you be 'illegal' ?" Or do you just help yourself, knowing that it's public land ?
Hence as you can see, you'd need to post more info, and even then: answers will vary on how skittish persons are (go figure, this is a family friendly forum too)