I went through a few batteries in my Mustang using a Battery Tender. I think it overcharged and boiled the water out of the battery. I moved to a different brand (Battery Minder) and have not had an issue. I also keep an eye on water level. The ultimate solution though, if you haven't done so already, is to switch the a sealed AGM battery and not worry about water. I have yet to find an AGM battery for my Mustang THAT I LIKE (meaning NO green, blue, red or round celled batteries.)

As for the mystery fluid under your car... I had a similar incidence with my Torino. Strange rusty looking water patches under the right rocker panel. Nowhere near any water lines. Not 100% sure, but believe it was spilled radiator coolant that leaked out or fell out while changing the radiator, then soaked into the concrete, and bled back up when time/temperature/underground water forced it back up. I laid a mirror over the wet patch to catch any drips to see where it was coming from, but later found the liquid UNDER the mirror. It actually ate the silver finish off of the mirror glass. It finally went away and has not returned.