IMO, your leak is bigger than a pinhole. When I drain the rainwater out of mine in the winter, it takes about 2 days to drop 6 inches siphoning through a garden hose. Look for something going through the pool wall at the level the water stops: Return, light etc. If nothing there, check the seams first.

If you have a main drain in the flloor, I'd be guessing you have a plumbing problem, maybe just a leaky valve.