I seem to have found what is responsible. The ECLSS Unit even when configured not to act as a monoprop fuel cell still acts as one. To reproduce, build a vessel containing a probe core, some oxygen tanks, empty nitrogen tanks, monoprop tank, empty water continer, battery with an ECLSS Unit configured any which way and it will slowly convert Mono and oxygen into water and nitrogen. Had four ECLSS Units hidden in the service bay of my docking ship and it would chew through oxygen trying to keep batteries up in the dark.