[Eldar14]

It was on the bottom of the Anduin for 2 and a half millenia ... and it still managed to resurface.

It was uder a mountain for 500 years ... and it still managed to see the light of day again.

You just give examples where the ring was lost ... for a while. Sauron will live forever as long as the ring is whole. He can wait.

Also, during both of these examples Sauron was in a state of regaining most of his power, and probably spent most of that time completely bodiless and powerless.

And in regards to the ring not being noticed on Frodo, that's kind of the whole reason armies worth of men and elves fought hopeless battle after hopeless battle; Sauron was distracted and had his eyes pointed outside of Mordor.

And as for the ring passing out of knowledge: Sauron never forgot about the ring. And as long as he remembers, it doesn't really matter if everyone else has forgotten.