Welcome to the forum. Would help to know the surface area and capacity of the pool in gallons. Rough estimates are fine. Reason for asking about the surface area is to get a better handle on the size of the leak. i.e. what does an inch of water mean in gallons. The conversion is about 7.5 gallons per cubic foot. If it is a pipe or joint leaking it would definitely be on the pressure side. If the plumbing is original you could very well have a crack somewhere due to ground settling and stressing a joint or pipe itself. Or, freeze damage? I'm thinking one weak spot would be around the return penetrations into the pool. One way to prove a leak in the return plumbing would be a pressure test using compressed air. If you feel comfortable messing with things then it really isn't that hard to do. If you want I can give you more details. You would have to blow the water out of the pipes as you would do for winterizing and you would need an air compressor of some sort. Even one of those small 12 volt things will work.
This is a start.
Al