It's evaporating. It's what water does. Many use a automatic top off ATO) of one sort or another. Others, myself included, add the water lost to evaporation manually. My 120DT/40sump setup loses ~ 2gallons/day. This is due to the dry heat from the furnace in the winter, and the AC condenser removing it from the air in the summer. It is perfectly normal for any body of water to attempt to equalize itself with it's surroundings. Leave a measuring cup full of water on the counter. It, too, will lose water to evaporation.