Welcome to the forum.
Just a thought on the 3/4" loss in 48 hours. That could very well be evaporation if you don't have a solar cover. As for the returns you can always plug the pipes and see if the level still drops. Something else to try is to wait until the water is really quiet and shoot a little dye at the returns and see if it gets sucked in. That would be the quickest test.
Al