Update:
Did bucket tests to determine that water was leaving pool much faster than was leaving bucket.
Reason salt level dropped so fast, so much, is that automatic water leveller was refilling lost salt water with fresh water, thus diluting salt.
Finally convinced pool builder that pool was indeed leaking, and I was going to retain final payment until fixed (as my right per contract).
They checked around, do some tests, and removed a fitting in pool, replastered around it, installed new fitting.
They pronounced it repaired. Please pay final payment.
Not so fast, says I, bucket test shows the water level in pool goes down 1/2 inch per day, instead of the previous 1 inch per day. At this time of year (winter), bucket water (bucket test) was dropping almost none per day.
They do some more tests, end up draining pool, replace and replaster all fittings in pool. Find a leak, I ask. Several, they say.
It has been about a week, and water level stable, not dropping (auto water leveller is turned off). Salt concentration is stable.
I breathe large sigh of relief that yard/pool does not have to get dug up. They are not getting paid until I'm sure it's fixed.
Many thanks to this forum for giving me the knowledge to use to convince pool builder. Looking forward to summer.