@Marcus_1234 welcome to the GPF! Glad you were able to find the "leak". As often as we hear "my pond is leaking" it generally comes down to plumbing or a low edge. EPDM liner is very tough stuff - if you have a hole or tear you probably know how it happened. The only exception might be an animal chewing through it, but that's not the norm either.
We have a low edge situation that I need to address but I kept thinking "there's no way this is creating the kind of water loss that we're seeing". I was standing staring at the pond Friday night, wondering "what is going on!". (My husband is the "we have a LEAK" guy, so I have to ignore his "suggestions".) Suddenly I realized I was hearing water trickling... in the storm drain... which is where our overflow drains. Our rain exchange basin needs a good clean out so it's not draining as fast as it normally does. It had gotten backed up enough that the pond was emptying itself right down the drain. EXCELLENT. My clue should have been that the water in the basin never went down. DURRRRR! Anyway, always nice to figure out a problem and have it be an easy fix!