I'm not disputing that but it will be evaporating from every where.how is the water entering the sump,is the supply pipework above the water level in your sump,if so this will cause splashing and help lower the water volume,is the sump in a stand under the tank,if so check the inside of the stand for any signs of dampness from the splashing,I had this problem when I first setup my sump, I also thought it may just be evaporation but it wasn't.