Water circulation, water keeps disappearing from recovery tanks?

There are only a few places that coolant can go in an engine. You don't see an external leak or coolant in the oil, which suggests that it is likely a bad head gasket and the coolant is being used that way. This shows a few tests that can be done to determine if that is the case:

https://www.2carpros.com/articles/head-gasket-blown-test

The pressure test and chemical test can usually be found as loaner tools at a parts store. I would suggest doing them and then you can decide what to do from there.