Our pool was re-plastered in April. Ever since, it was leaking, losing about 5/8" per day. We spent some time and effort searching for the leak, since the pool had previously leaked along the rock coping/plaster interface. We used epoxy on anything that even resembled a hole or gap. No use, still leaking.
It took some time to be certain it was not the waterfall or piping. We had a small piping leak at the pump and spent some time trying to repair that, creating a worse leak for awhile. Fixed that, not really successful but back to the very minor seepage it began as. That was not the leak.
Then when a light fixture fell out of the wall, we thought maybe it was the seal at the conduit that was leaking. Sure, I always read that the light is so often the problem, so nice that the light fell out so I would think of that. Dye test suggested that deep end light was not leaking. Couldn't get the light back in as the thing the screw goes into was not there and we were unable to get a wedge to work with so much cord to stuff in the niche. Finally hired someone to come seal the cord with butyle tape (not available in my pool store) and put the light back in correctly using the wedge we bought. Had him re-seal the cord in the shallow end light at the same time, just in case. Dye test there was.... not certain. Maybe.
American Leak Detection came out and found a small leak in the spa, at two returns. The plasterer came back and fixed that. But the pool was still leaking same as before.
Finally did a bucket test successfully to find that of the 5/8" per day, 3/8" was leak and 1/4" (at best) was evaporation. Killed about a week messing with that as the first bucket had been weighted down with rocks, but those rocks punched a hole in the bucket, oops. Ha! I'll use a METAL bucket.... sstupid bucket had a tiny hole in it too, from carrying too many rocks. After that long the pool was looking poor and shocking it with cal-hypo led to cloudy water that would not stop. Had to break down the cart filter a few times to clean it, what a pain.
After we got it clear, American Leak returned to find a leak at the shallow end light niche. It was really subtle. We think they missed it the first time because they were dye testing INSIDE the niche where the cord goes out the back. The leak was actually in the ring extension at the edge of the niche where the plaster company added a small ring to compensate for the buildup of plaster beyond the old face of the niche. I did not even know they had added such a thing, it was hard to notice, and I was told that the plaster would be at the same level as the original plaster.
I had a heck of a time trying to repeat the dye test but did see it take a bit of dye eventually. Even seeing the dye go behind the light (still in the wall as I did this) I never would have realized that this ring had been added. Then we left on a 2 week vacation, paying the neighbor boy handsomely to come check the skimmers and Polaris and spend an hour to add over an inch of water every other day to the pool to avoid burning out a pump with low water level.
When the plaster company returned and tried to repeat the test, their dye was heavier than water and did not seem to go into the seam but they felt that was it and so sealed the metal to metal connection.
Apparently it worked. Pool seems to be holding water now. (say a little prayer) I don't want to think about how much water we wasted, letting this go for so long, but we had vacations and things going on in the meanwhile.
So, if you have a mysterious leak on a re plastered pool, perhaps there was an extension added to the light niche that did not seal and was not sealed on the back side. Not something I'd ever read about.