A quick switch to single freq will also tell the tale on those "teens" bottlecap readings.
I was at a park today hunting for a gold ring (no luck). I decided to test something. Every time I hit a good, clear mid tone, I switched briefly to 10 kHz (which I had set as a "custom mode" under the user button). On rusty steel bottlecaps, most foil, etc., the ID would be substantially higher, and jumpy, in 10 kHz as compared to the way it read in Multi. Meanwhile, on a "good" target (like a nickel) plus a ring tab, rectangular tab, and those round foil lids from drink bottles, the ID stayed consistent/steady/nearly identical to the ID given in Multi. I will be testing much more with this, as I do think this will be a way to cut ALOT of trash digging. I have been aware of this "trick," for awhile now, but today was the first day I experimented with it heavily -- on every mid-tone target. This test seems to "weed out" things that are either not round/uniform in shape (like can slaw or irregular pieces of aluminum foil), OR those things that have some type of iron/steel in the alloy. Things that are of a better-quality (entirely non-ferrous) alloy, and are round or consistent in shape (like a nickel, or an aluminum pull tab) will give nearly identical ID in both Multi, and in single.
Steve