One of the things I noted when trying to electroform a Monotype matrix last May was that by the time I put that project on hold, half of my plating electrolyte had vanished. I was using an open tank so some of it may have been lost through evaporation.
In the intervening weeks I ran an informal test of the evaporation rate of water left open in my basement workshop, where I was doing the electroforming. Near the end of May, I filled an straight-sided open container partway with water, and marked the water level. Today I marked the water level again. Based on this time interval and the drop in water level, it seems that I should expect an open tank to lose about 5mm of depth per week.
In the particular tank I was using for electroforming, the surface area was 250cm² so this represents about 125ml of loss per week.
Given that altogether I lost 500ml in volume, and the tank was open for 3 or 4 weeks, that seems to correspond fairly well.
It should, however, be expected that a solution of electrolytes should have a somewhat lower evaporation rate than tap water so this still leaves the possibility that some of the water was lost through electrolysis. The temperature of the bath would have an effect as well. If the plating current heats up the bath that could speed evaporation. Unfortunately I never measured the bath temperature.