Is it quite possibly true that most adults in so named good health already have at least one positive cancer cell in their bodies that has yet to be identified and that it may never be?

I do not accept this idea at all. I would accept the idea that all have the potential to develop cancer, not that there is "one cell" hiding in readiness to appear. Believe what yo want. I know of no way researchers would test the hypothesis.

Not most as majority of individuals will never develop cancer, but some may be having early cancer that is hard to diagnose before growing enough to cause symptoms.