Filter causing a leak?

[twe]

I have a pool with a raised spa (approx. 8 ft diameter). My pool runs all day and is off at night. Every morning my spa drops between 2 and 3 inches of water. I have done several tests to try and isolate the problem.
When I plug the spa returns and drain I do not lose any water in the SPA.
When I turn the return valve in my pool for just the spa returns I do not lose any water.
When I close all of the suction and leave the pool and spa returns open I lose water.
When I plug my cleaner return and intake and leave the pool and spa returns open I lose water.
When I put the pool in recirculate mode I do not lose water. I tested this several times
I do not seem to be losing any water out of my waste pipe from my 6 port valve. I just changed the spider gasket because I thought that may have been the problem. I can not find any visible leaks anywhere near the equipment and the pool.
I tried using fix a leak to see if that would help and I am still losing water.
The filter pad sits level to the pool deck and the spa is about 2 feet higher than the pool deck
Could the water be slowly flowing back into the filter during the night if so how can I stop this or is this a leak and I have not been able to find it?

 

[OldPoolMan]

Do you know or have tested whether your pool is losing water overall? Most systems have a check valve somewhere on the pressure side of the pump usually after a heater that prevents backflow from the spa to the pool. If the check valve flapper is to fail then the spa drains down into the pool. Check for that type of valve in your system. Sometimes its pretty obvious because the spa water level will either drain down to the jets or equalize to the water level of the pool.

 

[twe]

You were right the check valve flapper gasket was leaking. Thanks for your help!

 

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OldPoolMan hits another home run!