Tracking down a potential leak

I'm trying to track down a leak that, so far, I can't figure out. The pool is rectangular 13' x 24' concrete that's about 45-50 years old. There is 1 skimmer and 1 return - both on the same 13' side and about 8' apart.
When I opened the pool for the year things seemed to be fine, although the day after i opened it (or maybe it was the day I opened it - it's a couple weeks ago) the water level dropped a LOT in a short period of time - way more than normal. I couldn't find any problems and it didn't happen again, until today.

The "only" thing that changed today is that I installed an eyeball (large size opening) in the return outlet. After that, the level dropped. With the amount of water I lost I'd expect to find a soggy spot on my lawn, or some sign of water somewhere, but I didn't.

The return isn't the original one that was installed with the pool, or it's been repaired at some point, because the deck concrete over where (I assume) the return piping is has obviously been replaced and has sunken some (maybe 3/4". I don't think it's sunk enough to have wrecked the pipe.

I'm also assuming that it's not a suction side leak sine I'm not getting any bubbling in the return water unless the water level drops enough for the skimmer intake to run low.

One additional note is that the main drain in the pool has been cemented shut.

I've currently got the pump shut off, and haven't lost any more water. I'm checking the evaporation rate w/ a bucket on the steps but don't think that's it since the water loss isn't happening overnight. I've also checked any suspected spots on the walls w/ dye and can't find any wall leaks.

At this point I'm looking for ideas on figuring out what's going on without digging up the lawn or concrete.

All thoughts are welcome! My sincere thanks for any help.