If there is no free water and emulsified water present in the oil, than this increase is most likely due to high solvency of water in this oil. I have seen cases where a vacuum dehydrated mineral-based ISO 100 turbine oils (Group 1) increases moisture content between 500-800 ppm in short time while being stored in a storage tank (with saturation point certainly being even higher because there was no evidence of free or emulsified water present).
It has been noted in some literature that vegetable oils have higher tendency to dissolve water than mineral-based oils and synthetics. That’s why a vacuum dehydrated oil when moved to storage tank (without desiccant breathers installed) will tend to regain their natural oil-water equilibrium.
The additives added (to any oil) due to their polarity, they increase water solubility in such oil. That’s why gear oils have more affinity to dissolve water than most less additized oils (i.e. most hydraulic oils, turbine and transformer oils). Oxidation by-products (polar varnish particles) also increase this solubility, and in this case, vegetable oils are more vulnerable because they are more prone to oxidation than mineral-based oils, thus having a shorter service life.
You can do your own experiment, by sending a sample of dehydrated oil to a lab for testing (immediately after dehydration process). Keep a portion of that oil on your desk in a big uncapped glass graduated cylinder (or some transparent container), and send a portion of that oil every day for testing. Just by observation (and the test data) you can spot the point when the oil reaches its water saturation point by releasing the first water droplet. Knowing this, and by evaluating those test data you could be able to find out water saturation point of your oil. This is not a scientific approach per say, but rather a way to gain useful understanding of your oil’s characteristics.
