Metal detectors, do just that, detect metallic objects. It can't really tell the difference with all certainty. It can give you some calculated guesses about type of metal, size, shape, and depth. These guesses are based on known objects, and in ideal conditions, not exactly real world stuff. Works pretty well for most folks, since it will lock into information that almost exactly matches what's programmed, which tells you with some certainty. Most of the time, it gives you the best information it can, and you decide if you want to take a chance. Sometimes, you really don't want to dig dozens of holes, in a small space, in the same hunt.
When I hear the beep, I go back over the place, slow, steady, and even. Give the machine the most reliable chance of reading the target. I try different directions over the target, to pinpoint it's location, see if it's got some size to it, and see how the numbers and sound changes.
Metal detectors only have so much range (depth). The can usually detect objects much deeper, but the signal is very weak. The depth and ID does get enough information to make a guess. Digging deep isn't the most fun part of the hobby, unless you know there is a pretty good chance of a great reward for your efforts.
One beep, for twenty swings, I'd just pass on. It's either tiny, or really deep, no real information to get me curious enough to dig, not enough beeps to pinpoint where to dig a reasonable sized hole.