writes
if someone 'owned' him and let him rove freely, un-neutered, and
let the mites get to the point where his ears were running, should I
call them and tell them I found their cat?

If someone let their cat out without spaying/neutering her/him first...I'd say,
they lost any rights or courtesy in "notifying" them about the whereabouts of
said cat. If the cat acccidently ran outside, then, that's another
tale......Perhaps they were treating the cat (for the earmites) before he
"accidently got out"?