Not just possible, it is a fact.
It is a slow process, but the Tygon, and other brands, of tubing we use in WC actually absorbs a small amount of water from inside the loop. It then evaporates from the outer walls of the tubing.
The more tubing in your loop, the more noticeable it is over time.
Heat and pressure also speeds up the process.
A system like mine with 2 series DDC pumps in a long loop, CPU + 2xGPU folding 24/7 in relatively high ambient temperatures, will loose water much faster than a CPU only loop that is only on part of the day.
If you want completely non permeable tubing, you need something besides flexible PVC tubing.
The really noticeable drop in water level in the first couple of days or weeks of running is most likely air bubbles making their way to the reservoir though.