I got to thinking about why there are 4 drain openings and realized that the pool builder might have dedicated 2 of them to our dry hydrant. (A dry hydrant allows the fire department to pull water from the pool directly.
The line to the dry hydrant is like any other drain line except that it connects to a hydrant head above the level of the pool water surface. If the pipe to the hydrant breaks breaks below the pool level, the water will drain to the level of the break. In my case, it broke below the bottom of the pool.
In retrospect, the dry hydrant was completely unnecessary. I talked to the local fire chief and he said they never use them. It's faster to use their own pump.
Our house is in the middle of 200 acres. We have 6000 gal of water in storage with a fire hydrant near the house. The local fire chief said if they go through that and they use up the 20000 gal in the pool there probably wouldn’t be any house left to save.
I had a similar situation once. I had a handyman helping finish off a list of tasks before I sold the house. He did a backwash on teh pool and turned the pump off but didn not move the multiport valve to the filter position, leaving it in backwash. The pool was much higher than the equipment pad and after he left the pool slowly draind theru the filter.
I came home late that night to find the pool about 2/3 (maybe 1/2) empty.
The first feeling was shock, then panic trying to figure out what happened. I stood staring at the equipment pad for what seemed like hours, but was just a couple of minutes. Then I saw it.....
It took the better part of 3 days to refill the pool from the hose.