Ok... I plugged the main drain for 24 hours using one of those PVC plugs with the wing nut on it that I got at Home Cheapo, and still I have over 1/2 of water loss.

So much for my theory that main drain was leaking and the crete was put their on purpose.

I'll plug the skimmer also and see if that slows it down.

Speaking of "slow" --- a slow process of elimination, huh?