Looking back at my last post here I think I'm partially wrong. What I was suggesting was a leak in the MD pipe at some lower level, maybe near the bottom of the pool. My comment about the leak rate would be correct only if the MD was somehow capped so water could not flow into the MD pipe via the MD itself. Maybe that's what they did...cap the thing somehow. I have no experience with main drains....and don't want any. Dye and being able to isolate lines should point you in the right direction.
Al