Seems totally normal to me. If we get 10% humidity in these parts we call it "rain."
It's not going to take a long time at all to lose heated/open surface water like that. Indoors, I lose 4-5 gallons per TANK per day in a house maintained at >60% humidity, so 15 gallons in 3 days, outdorrs and heated, at this time of year .... I'm not surprised at all by that kind of rate. In fact I bet the loss could be a lot higher than that if there was a lot of localized air movement over the water surface...
Just take a look at the trees around Banff Springs on a day below -15C .... that's a lot of water ripped out of that pool.