I had something similar to the OP but I was losing water at a rate of 1 inch per day. My raised spa would drain to the height of the returns every night and I could not figure out why. I looked at all my equipment and could not figure out where the water was going. I had a crack in my Pentair whisperflo pump volute & housing so I replaced that myself. That was not leaking enough to lose 1 inch of water per day so I knew that could not be the source of my problems so I looked elsewhere. I turned my spa on to heat up on a cold night and the spa would drain to the returns after 20 minutes. The spa jets would be shooting water out of the pool. I was very worried at this point and I knew I had a bigger problem on my hands.
My troubleshooting began in the spa light housing. I shut off the pool completely and sealed up the conduit to the spa light and waited over night. The water would still drain to the height of the spa returns eliminating the spa light being the culprit. I then capped every hole in the spa and filled it up with a garden hose. That confirmed I did not have a leak in the plaster as it did not leak. I then pulled the plugs on the spa suction drains and water supply on the bottom of the spa and it did not leak leaving the only item left are the spa returns. At this point, I had nothing plugged in the spa and returned the next morning to see the water level dropped to the height of the spa returns confirming I had a leak in that line specifically.
I took off the spa qt blower and glued on a 2 inch pvc tee so I can put on a pressure gauge and a connection for my water hose as well as a connection for my air compressor. I plugged the 4 spa returns with inflatable rubber test plugs. Then I proceeded to fill the line with water then air to 10 psi and it would drop rapidly confirming I indeed did have a leak in the line between the pool equipment and the spa.
I do not have any high tech equipment so my tool of choice was a styrofoam cup with a hole in the top. After hours of wasting water trying to keep the line pressurized at 10 psi, my idea of laying down on the concrete using a styrofoam cup listening for leaking water paid off as I saw a small crack with a very small amount of water seeping through it. It wont let me attach more than 1 picture so I cant show you the leak that I spotted on the concrete decking. After seeing the water, I began to drill a series of 1/4 inch holes in the concrete to see if I can hear water seeping out of the line. BULLSEYE!!!! I not only heard water leaking, but I also saw the water coming out of the hole. I drilled several more holes in the vicinity and determined that is where I had a leak. I saw cut a hole in the concrete and found very wet soil but no pipe. I was off by about a foot so I decided to saw cut a 2x2 hole in the concrete and found the leak. I shut the compressor and water hose off then began to dig out all the dirty water and mud so I can see the crack. It was a leak in the 45 degree elbow going straight to the spa returns. I fixed the leak by using the 4 90's method and waited for it to dry until the next day. After about 24 hours of it drying, I turned on the spa to fill it up then left it over night and woke up the next morning to a full spa. After battling this problem for a few weeks, I enjoyed a couple of cold beers sitting inside the spa watching the sunset.
I would love to post all my pictures but it wont let me probably because I'm a newbie. If a moderator can help me post some pics, that would be great.
Not really, it's ALOT over my head.