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I'm thinking that you don't have a leak. What i suspect is that the water on the cover and the block of ice in the pool are pushing pool water out (I might be able to guess where if I knew more about your set up).

Your experience from last year verifies that the pool isn't leaking - or, at least wasn't last year when the same thing happened.

What I suggest is to measure down to the water level and keep note of how far down the water was upon opening. Then go ahead and refill the pool and double check for a leak with the bucket test. If the bucket test shows a leak, you already know approximately how deep the leak should be.