on first & 3rd diagnosis. bad radiator-cap is mostly the cause for minor loss of water like this case.
By the way, sometimes the escape of water/coolant from cooling system can really make one's head go nuts.
I am facing an identical situation. I serviced my radiator, changed radiator cap, however, everyday there is still some loss of water (half a glass) from the reservoir bottle. Car is running fine, good economy, no oil trace in radiator, no smoke from exhaust except minor smoke for a few minutes during cold starts in harsh winters (december/ january). The loss of water remains unaccounted for.