To add to what Frank said (and possibly to clarify what Mr. Dankowski observed), sometimes a very small piece of junk will obscure a larger target. If that piece of junk as been discriminated out, it's possible for the obscured target to disappear as well. In the case of a machine that doesn't null on discriminated objects, the result will be complete silence. You won't know that you passed over an iffy target because there won't have been an iffy target, just silence. It's easy to find Youtube videos showing machines hitting on targets with junk directly beside or even on top of them, but the videos of machines finding targets with the junk a few inches above them are less common. There's a reason for that.
Some machines have a reputation (deserved or not) for winkling those targets out, but even then, it's possible to miss things. The only way to be absolutely sure that you didn't pass anything up is to dig everything in all metal. You probably don't want to do this. On public land, I'd argue that you should never do this, at least not all at one time. If someone was really interested in truly cleaning up an old park (and I've thought about it myself), the proper way to do it would be to cherry pick, then drop down to zincs, then drop down to foil, and then finally drop down to iron, spacing each trip out by a few months or so to let the turf recover. Finally, you would hammer it with a PI machine; depending on the amount of tiny junk recovered, you might have to repeat the process again after that.
I'm not sure that I have the patience for this approach, but if I wanted to call a location truly and completely hammered, this is how I'd do it. Even then, it's possible that a different machine might find a thing or two, or approaching from another direction with the same machine might turn up a few more items. Such are the vagueries associated with metal detectors.
Mr. Dankowski has written several articles about this phenomenon, complete with experiments in the field. You'll find them on his website. Suffice it to say that when a place has been "cleaned out," there may be more there. The question is whether your time is best used in finding them, or finding a better place to hunt.
