Interesting topic. Maybe CA is just more litigical, but you can here ask till you're blue in the face here, and you'll rarely get permission to go anywhere. All demolition sites are fenced now. I remember when they were wide open when I first got into this hobby. Mind you, it's not that any contractor or property owner cares that someone might cart off a wheat penny (that's about to be covered by 5 ft. of fill, or hauled away in trucks, or whatever), it's only fenced because no one wants to get sued if you slip on a banana peel. And the minute some demolition contractor says "sure, help yourself", he can get sued if you get hurt. So we/I merely wait till after 5pm, and help myself.
An example was when the local public high school, built 1919, underwent rennovations some years back. The inner grass court, where hundreds of kids have eaten lunch for 70+ yrs, was slated to be scraped out. As usual, when the demolition started, they put the obligatory fence around it. Someone from out club tried and tried to get permission from the city bureaucracy, to get in there and metal detect. They got a "no". Imagine their surprise, when they show up at the next club meeting, to see fistfuls of silver on the display table, by those of us that just helped themselves (the gate was merely bailing wire shut). Mind you, we were in full view of Main St. traffic, and afterschool personell merely waved hello. One janitor came by night after night to see our latest finds, and never said a thing about fences or whatever. Sure, maybe this public school example was a bit more innocuous than 1) a non-public demo, or 2) an private abandoned ruin, etc... But I think it makes a point that if you ask around long enough, someone's gonna tell you that you can't even pick your own nose.
Ivan and ThT2000, whenever this subject comes up, someone's bound to rail on about jail and confiscated detectors. Be honest, when was the last time you ever heard of an md'r get arrested, confiscations, etc....? I suppose if you can cite an example, it was no doubt someone who was being a nuisance, couldn't take a warning, and was night-sneaking sensitive historical stuff. And if you CAN cite an instance of someone getting ruffed up for something otherwise, I would say that's the rare exception. I mean, afterall, sometimes a policeman roughs up a guy, pepper-sprays him, etc... a for a tail-light infraction. Does that stop you from driving?