Where is my water going? No apparent leak

[know-nothing]

Every 3 to 4 days I had to refill the pool as the water level was dropping below the skimmer.
I noticed that after the system had stopped. The water in the spa was dropping down from the over-spill level by several inches in matter of minutes.
The next day when the system started again the spa would fill up and the same thing happened. When the system starts then they are air bubbles coming out from the returns for a few minutes. Eventually the water level would drop below the skimmer. There is no evidence of leak on the equipment pad.
I marked the water level and turned the system off for 3 days. No change in water level, in the pool or in the spa. That ruled out a leak in the pool itself or the spa itself.
I found a 3 way valve and closed the spa return so that all of the water was going thru the pool returns. The pool level was then stable. So I thought I narrowed it down to the spa.
But I noticed something interesting. Although the spa return is closed, the water keeps dropping in it while the pool level is stable. Again, turning the system off completely leads to no level drop in either spa or pool.

Where is my water going? Why only when the system is being pressured? Why even when the spa is return is shutoff its water its water disappears?

:?: :?: :?:

 

[JasonLion]

You have a plumbing leak above the pool water line. It sounds very much like it is in one of the spa lines, but that is less clear.

Have you done a through visual inspection of all of the exposed plumbing while the system is running to see if you can spot water coming out anywhere?

 

[spishex]

You should have valves to differentiate between the two systems on both the suction and return sides. If you close off just the spa returns but leave the spa suction open you will pull water from the spa and put it into the pool, thus draining the spa. Are you changing both valves or just the return side?

 

[anonapersona]

You could also have a leaking check valve between the spa and the pool. I had that and it really complicated locating the leak in the pool. In our case, the spa water flowed back into the pool, raising the pool level, so the bucket test was not all that effective. Not the answer, but can explain part of the spa level change. Does the pool level go up when the spa goes down?

Long story of our search for the leak is on a post titled "Loss of 3/16 inch per day" or something like that.

Short story is that I called American Leak Detection, spent $400 to locate the leaks and $20 to fix it ourselves. They tested all lines and found them sound, but for the leaking check valve and a tiny leak in waterfall rocks that was not significant to the pool level. Two more visits found leaks under the rocks edging the pool. Fixed with two part epoxy, a sort of pool putty that works underwater.

 

[the.peoples.champ]

We also had the same problem with refilling the pool. I checked all of the blumbing that was visible (above the ground) and I found the the waste line that went to the side of the house and was piped in to the sewage was leaking. Lucky for me the sewage pipe had a removeable cap otherwise I never would have found it. I could tell it was leaking because the poool was off and there was water falling into the sewage line and I could hear it. I later found it was the spider gasket of the multiport valve. I replaced it and lubed it up and have had no problems ever since. Just a suggestion.