As a minor sub-plot point, in the Arthur C. Clarke book "Light of other days"
There was the discovery that prior to 'snowball Earth' there was a sentient civilization of crustaceans in the ocean...
I like to flesh out sci-fi story ideas in my head (almost none have found there way onto paper) and when I was younger I played around with the idea of a sentient species becoming technologically advanced ~66million years ago on earth... but could find no plausible way to believably explain why their traces had not been discovered by humans...
From your statements you seem to think the evidence might be easy to miss after tens of millions of years?