Leaking pool. Help!

My 6 year old above ground pool is leaking at a rate of about 500 gallon every two days. We thought the leak might possibly be at the skimmer/water return area because that is where the water finally came to rest. We took out the underwater light and put in a new eyball return with brand new gaskets. We tightened the screws to the skimmer and caulked around it. per advice from the local pool lady. We have even used the dye testing method and everything we try is inconclusive, because the pool is still leaking. It doesn't leak anymore or less with the filter on or off and after it reaches the midway point on the return it leaks no more. but of course at this point our filter is running dry and must be either refilled with water or the pump shut off, so u can imagine what our water looks like. We have done a visual scan of the liner around the perimeter of the pool and cannot see any holes or punctures at or near where the possible leak would have to be. I might mention also that we don't see any wet spots that might indicate where the leak might be. Please any advice or help would be greatly appreciated.