I would like to say one thing, that sometimes gets forgotten in these threads.
Where you live, matters.
What I mean, is this -- I grew up in western Pennsylvania. I am also a hunter; deer, small game, you name it. Now, in Pennsylvania, it's a rather common practice during the fall hunting season to simply drive around, look for a nice patch of woods, and then get out and hunt. If you live there, you know exactly what I mean. The rule of thumb is, if there is not a "posted, no trespassing" sign, then of COURSE you can hunt there. Property owners know it, hunters know it, EVERYONE knows it. IF, for whatever reason, a landowner does not want anyone on his land (and it's rare), he KNOWS that it is HIS RESPONSIBILITY to put up no trespassing signs. Otherwise, it's just a given that anyone can use the land for hunting. Now, if you are wanting to hunt an active farm, anywhere near the barn or homes, most people will ask the farmer first. But just a patch of woods or an unoccupied field? It's fair game, and again, EVERYONE knows it. It's just kind of understood, a custom. No one -- hunter OR property owner, ever gives it a second thought. SO, metal detecting in that part of Pennsylvania is much the same. If it's out in the country, a patch of woods, no homes nearby, NO ONE CARES. Again, it's just the local custom in that area.
ON THE OTHER HAND, my first move away from PA after college was to move to Texas. One of my first questions, upon arrival, to the guys at the office was, "hey, where is a good place around here that I might go deer hunting?" They all just LAUGHED at me. "You can't; you need a lease." A what? You mean a hunting license? No -- a LEASE! I had no idea what they were talking about! Over time, I came to understand -- property in Texas (and in Oklahoma, where I live now...and across much of the "West") is OFF LIMITS, without express permission from the owner. People out here have a TOTALLY different attitude toward property, both the owner of the property, and those who wish to use said property (for hunting, or fishing, or whatever). I can't even begin to describe how different the local customs are here, compared to back home.
SO -- my long-winded point is this. I see that the one person raising the most objection to hunting without permission is from Texas. And I GET THAT, from his perspective (now that I have lived in this part of the country for over 20 years). But, what gets lost in these threads is that customs are different -- VERY different, in different parts of the country. A person in Pennsylvania would LAUGH at the absurd idea of going through the trouble to find out who owns this little piece of forest or field out in the middle of nowhere, just to ask permission to take a walk with their detector. Because ABSOLUTELY NO ONE cares! It's NO PROBLEM! On the other hand, someone in Texas might likewise laugh -- equally hard -- at the absurd idea that anyone would even THINK you could take a walk through a forest or field for ANY reason, without SPECIFIC PERMISSION from the landowner.
There is no universal "answer." That's, I think, why some of the disagreements occur in these threads, that BBsGAL referred to earlier. We all might keep in mind that where you are raised, and the local customs, play into our perspectives each time we respond to posts such as this one. There is no universal answer, with respect to private property, or even public property.
From this point of view, Tom in CA's perspective on permission, etc. would make total, complete, logical, obvious sense to most western Pennsylvanians (myself included). Meanwile, CJ319's perspective would likewise be "a given," an entirely correct and rational point of view, that would totally resonate with most Texans. Having lived in both places, I can completely see why some of these threads end up in disagreements...
Just my two cents...
Steve