pool's main drain is probably the source of leak. any suggestions??

I have an inground pool, and we have had a pretty steady leak the entire winter. We had a guy come out and give me an initial look, and he ruled out a leak in the return line since the water keeps dropping. He suggested that it is either a leak in the pool wall or at the main drain at the deep end. I have come to think it is the main drain because the water will not level off. so does anyone know, or have any insight on what that would cost to repair? He is coming out next Monday to figure out what else to do. t is a concrete pool. it has been closed the entire winter, and the filter and pump systems have not been working. it isnt a leak in the pump and filter because whether that is running or not, the leak is still present. so any ideas? The guy suggested plugging the main drain and investing in a cleaning system? Thoughts?
I have also heard that sometimes there is a valve that regulates air on the bottom and that may or may not be related? I was first told that if it was the main drain it may just need some adjustment. who knows? Does anyone have any good news to add? or anyone with the same thing happen that just used a robotic cleaner?
THanks
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Have you tried plugging the main drain and seeing if the level continues to drop?
You have confirmed that the drop continues below the light niches and any other fixtures in the walls?
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