I have a couple of questions.

Do you see any soft damp wet spots on the ground anywhere around the pool? Say a soft spot exists near the back corner, that would be a pretty good clue you have a leak in that area.

Have any trees or bushes mysteriously gone dead? I discovered a leak that way once. A leak in the bottom got to a spruce tree.

Did you have any limbs fall on the cover during the winter, like say some ice caused some limbs to fall on the cover. That happened to me two winters ago. It wasn't until June that I noticed the level falling and discovered two small tears in the liner wall right where the limbs had fallen.

Good luck.