I've done training seminars related to detecting. This question comes up in EVERY seminar. Different detectors have better, or worser (ha!, not a word) capabilities.
A larger, solid, heavy mass item often can be detected deeper than a small item such as a dime, and a dime laying flat can often be detected deeper than one that's buried in a vertical position.
My detector does have a depth gauge, and I use it to determine depth of course, but also on certain sites to weed out items lying closer to the surface in an attempt to dig deeper items that may be older.
Often times too, items will only work their way down a few inches, some times this is related to the weight of the object. Most items I did in North America are between 2 and 6 inches deep.
Your question as to whether leaves count in determining depth, I'd say for the most part, yes. If I hear a signal, I'll often kick the leaf matter away, so as to get a better hearing on that signal. Leaf matter on the forest floor can often be quite thick, 4 inches or more. If I hear a signal in the ground, and raise my coil to knee height, I often cannot hear that signal any more.
And yes, assuming they detected the area, it's likely they dug other stuff, probably a couple beer tabs, bottle caps, maybe a spent rifle shell from years gone by, wadded up aluminum foil, rusted nails or who knows what. A recently ejected bullet would be an obvious sound in my detector, loud and clear, and it would also be obvious by the coloring of the brass as to approximately how long it had been there.
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