Condensation. I've seen garters take advantage of condensation. It has to do with the temperature and humidity of course. It's conceivable that a snake (especially if there is a localized heat source) can warm themselves up, and move to a cooler, humid area of the tank, OR cool themselves down and wait for the temperature to go up, resulting in condensation forming on their bodies. I've seen it before. A soaking wet snake, with no water around. The water came from the air!

Think about it. How did that cold glass get so wet that it is dripping water from it's surface? The answer is that the glass is cold, and the surrounding air is warm, and humid. works in reverse too. Warm snake, cool moist air.