for any number of reasons like voltage drop somone MAY have run a 12
gauge wire part way thru the circuit ..............

This is true, but it can be a reasonable assumption if you have #12 at
the outlet (in and out). You can then check the panel and if it has
#12 you can be reasonably sure.

I SUGGEST someone check EVERY outlet and fixture on that circuit!

People do all sorts of wird things. I know someone wh combined many
circuits in work boxes to save some ground wires

better safe than sorry.