That is strange the temp difference could make some difference in water levels,the level may drop a bit when the water is cooler and rise when it gets hotter, from the expansion and contraction of the water. Other wise I would think some one is putting water in the sump from time to time.. The ATO should be able to be adjusted so it does not add too much water. What wattage are those MH's if they are 400watters then they may cause extra evaporation. What size of tank is it and what are the dimensions, a tank with a larger surface area will have more evaporation even if it is the same size as a tank with smaller surface area.

Bill