Nuances of acid testing sterling silver

I didn’t see this covered in any threads over the years on tests for
sterling silver, so I thought I’d ask the experts. I regularly use
the scratch stone and acid method to test pawn jewelry we get in for
the store. I keep a piece of known sterling silver in my kit to
smear down a strip next to a suspect smear if I have a doubt.

I’ve found that when I put a drop of the red testing solution onto a
smear of sterling silver the smear shows as rich reddish brown and
the smear stays intact on the test stone. When testing some items
the color is similar to sterling silver but the smear of metal on the
stone dissolves and disappears. My question is, what is the silver
content, if any, of a metal that shows this kind of result?