My loop held water tight for years, last couple years water loss has slowly increased to where I need to find it. I'm losing about 1/4 cup in a week or two of very casual use. I've tried a couple times to spot it without tearing into it. I find no sign anywhere. The cap on my XSPC res has always vented a bit, but I doubt water is escaping there. Everything is bone dusty dry & the water has always been clear. Thought I'd ask here before I start taking everything apart.

EVGATech_MDecker
you can add UV coolant to the loop and use a black light to see the leak points.

Brilliant! Thanks. Any reason I can't use the $5 automotive stuff? I'll flush it all out after I find the culprit.

If you have any venting evaporation is the likely cause. Next in line would be an air pocket in the loop that shook loose. Have you moved it lately?  If you don't see a puddle somewhere it's not a bad leak. Very small leaks will evaporate before creating a puddle and may not be cause for concern for years unless it gets worse. 

owcraftsman
If you have any venting evaporation is the likely cause. Next in line would be an air pocket in the loop that shook loose. Have you moved it lately?  If you don't see a puddle somewhere it's not a bad leak. Very small leaks will evaporate before creating a puddle and may not be cause for concern for years unless it gets worse. 

        
 Evaporation could be the problem, I just want to make sure it's only through the vent. The loss seems to have gotten worse over time, especially the last few months. I have added a half ounce of the auto dye & will let it run until tonight when it's dark & I can see better with the UV light.
 

I'd tend to think it's evaporation, as well.  I had something similar recently, and there wasn't a leak anywhere.  Plenty of non-leaking evap points exist in many loops, from joints in rotary fittings, the cap on the reservoir, and plenty of other places that just have enough space for the pressure to blow off...literally.

Glad it was nothing major

OP,
Something to think about. You indicated that it was getting worse. You could have a cooling system that is in need of attention. Both inside and outside.
 
Venting is caused by heating of the coolant. If your cooling system has internal restriction (usually in the radiator) it will run warmer. Also, if your radiator fins are filled with debris it will restrict the air flow through it causing it to run warm or worse. Insure your radiator fans are working properly. In either case your coolant will run warmer and expand as well as vent. Make sure when filling your cooling system that you don't overfill it. Allow for the coolant to expand. Also, if possible, check your coolant cap to see where the pressure is. I would check with the MFG to see if there is a cap available for your unit.