Block off the skimmers and see if it still leaks with the pump shut down, if it does then it is most likely the liner or main drain. If it does not it is either a leak in the skimmer, the return lines or the feed lines. Close one off at a time and run the pump for 24 hours and see which one causes the water to drop. I recently had a problem like this and found it was the pressure line feeding my polaris that had a crack in it under ground.