When did you last refill the loop?
I have found (although it's been a while) that the first 6 months the fluid level falls then it levels out. This could be either all the air pockets getting slowly filled or some evaporation. Then it levels off.
Ran my last loop fluid for over 6 years! Levels didn't change after the first 6 months.
It was only last weekend actually TBH, I redesigned the layout, I went from 7 slot config to 5 slot config, the case is a Lian Li O11 mini air, I added a 240mm to the side intake instead of a 120mm with the pump/res, which meant the only place the pump / res could go was the back of the case, however, thats looking at a window now (you can see the blinds behind) which the sun beams in through in the afternoon.
I had to mod the back of the case a little to get the pump through, I could have used the mounts that came with the res but didnt want it sticking inside the case another 30mm or so, basically the pump is outside the case.
I wanted to get the 280mm rad off the bottom of the case which was throwing hot air in this weather into the case, ive replaced it with 3 x 120mm slim fans and moved the 280mm to the top.