Major pool leak

[Poolusernc]

I have a vinyl lined inground pool 18 by 40 feet. We use an elephant cover during the winter and keep it running 4 hours per day. It has an on/off sensor for cold weather as well. Pool liner is about 10 years old. Plumbing and pool about 30 years old. Came with the house. Over the winter, the pool mysteriously drained to about 2 feet from the bottom of the deep end. We were thinking of filling in the pool because we figured it must be an underground plumbing issue so we punctured holes in the liner to remove it. We then realized that the liner was very easy to puncture and very sun damaged and thought perhaps we could save the pool with a new liner. We do not, however, want to purchase and install a new liner if it's the plumbing (which we would not wish to fix since it is so far underground). Is there any way to tell which it was at this point? No visible liner tears or holes other than what we made. No hydrostatic valve. One drain, one skimmer.

[Teapot]

Get someone out to pressure test the lines and give them the thumbs up or down.
Liner pin holes can be present when the pool is full as the liner is at stretch but after a certain amount of water has drained the liner retracts and can close the hole, this does not necessarily happen at the height of the hole.
For example, a customer had a leak where the floor and wall meet, however the water would stop leaking at the halfway point this confused the regular pool companies they called out but I located the leak quickly with the Anderson leak trac.