Water disappears fast, but stops at a certain level

Help please.
Location North Florida, decent rain
All through winter there has been relatively low humidity and only mild rain.
I am doing back yard work so I've only been letting rain fill the pool since dirt and debris gets in it anyway from the yard work.
All through winter the pool has stayed about half full.
Once a week I treat the water to keep algae growth away.

One week ago we had our first big rain. The pool water level rose about 3 inches after the rain.
And then it dropped back down to the previous level in a matter of days...and then it stopped losing water again. ????
It's almost as if there is an overflow opening somewhere keeping the pool water level half full.
Am I supposed to be closing the knob valves somewhere after turning on the filter to prevent overflow loss or something?
The only openings in the pool are literally the drain at the bottom of the pool (if that leaked then all the water would drain out right?) and the opening for the light in the side of the pool...hmm...which happens to be right at the fixed water level now that I think about it...

Does that happen? Water leaking out through the lighting fixture??

Thanks

1) The size of your pool in gallons ... 20k
2) The type filter you have (sand, DE, cartridge)... Sand
3) If your pool is an AG (above ground) or IG (in ground)... IN Ground
4) If it's IG, tell us if it's vinyl, plaster/pebble, or fiberglass... Some kind of rough cement recently recoated with Olympic Zeron, which is freaking fantastic...algae can't hold onto it very well...I only left the steps the original rough material to avoid slipping.
 
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