one guy, I'm old enough (52) to remember when demolition sites never had fences around them. Oh sure, ... perhaps some ribbon and a few orange cones (that you just stepped over), but that was it. But nowadays, d/t the litigical environment we live in (people wanting to sue for slipping on bananna peels), they started putting fences up around barren dirt, ...doh! Also theft of const. equipment and supplies is an issue (recycle metal values, etc...) which is also the cause of fences. So ... nothing to do with metal detectors, IMHO, as their origin or purpose.
I have rarely ever seen any that were "patrolled", as you say. Most of them you can stil get in, by simply un-doing the bailing wire that holds the gate panels shut .... and presto, just walk right in. Or get a cat-key off ebay, as a lot of times it's just a simple cat-lock on the gate. Or if it's a combo. lock, that's easy too: Just look at it during the day when it's open (using a little due discretion, of course), and ... odds are .... whenever they've opened it that morning, the person leaves the #'s on the lock (fails to spin the #'s). Then when you come back later, just enter in those #'s you saw during the day. Presto
You say you can't drive at night. So what's to have stopped you from going on Sunday when no work is going on? I mean, .... if you say others are getting old coins there now .... how are THEY doing it? How are THEY getting in?