Pool unexpectedly drained (significantly!)

[lostinhydration]

New to the world of pool ownership. My partner and I I purchased an older home with a fiberglass in ground pool (33 k gallons~).

Context:
When we moved in late last summer the previous owner was using biguinide, however, due to excessive algae problems my partner switched to chlorine prior to closing the pool over winter.
We recently had a major storm and flood which came just short of entirely filling the pool cover. We pumped this out, and re-evaluated the possibility of an early opening for pool.

My partner prepped, and we filled the pool, backwashed, rinsed, etc. and decided to allow the filter to run over night. In the morning everything looked great, so the filter was turned off.

I checked on the pool about 6 hours later to find the water level to just above the drains.

(Other caveats: pressure gauge is at a baseline reading of 25 when off. When it was on it was running about 35. )

There were no signs of where the water drained, all around the house and near the backwash area.

The previous forum on this subject gave a lot to consider, but no new posts in over 10 years.
Hoping this isn’t a shout into the void!

 

[PoolStored]

Can you post pictures of the pool so we can see the water level, and post some pictures of your equipment pad.

 

[lostinhydration]

Yes!
The first image is current level. The latter photos are at original capacity. The water volume of water was drained to (?) over the course of 6 hours.
I have noticed that the gaskets around the return jets will need to be re-sealed, as well as the liner....though there is still the issue of the pressure.

 

[PoolStored]

Is there pool protrusions at the current water level? Post a picture of that.

 

[lostinhydration]

The liner does not appear to be protruded at all.

 

[PoolStored]

Sorry, language. The pool water will fall to the level of the source of the leak. Protrusion means "return" or suction port or some other "protrusion" through the liner. Inspect the liner at the pool level for any cuts or protrusions...

 

[mknauss]

Where does the backwash line go to?

 

[PoolStored]

Are you sure the pump was turned off during the time the water level lowered...

 

[lostinhydration]

It leads out and away from the house, away from the filter

 

[mknauss]

If there is a leak in the spider gasket of your MPV, it can siphon water out the backwash line. Can you see where the backwash line exits?

 

[lostinhydration]

If there is a leak in the spider gasket of your MPV, it can siphon water out the backwash line. Can you see where the backwash line exits?

Yes
It was checked after the draining as well