Well I'm no expert but I did have a similar scenario the year before last. Opened in the spring to find the water level way way down. Thought I might have a leak too but added water to the pool upon opening and watched the level closely after opening. No leak whatsoever.

I noticed just a week os ago my water level has slipped below the skimmer once again but I had no leak issues this past season.

My theory, and it's only that, is that environmental conditions are variable and at times this factor must increase evaporation. I'd guess warm and sunny days are the culprit. Sure enough two years ago we had little snow during the winter and fall this year has been super warm and mild.