Most of the time people are putting water into their gas cans and don't even know. Ethanol in fuel contains a certain percentage of water left from the manufacturing process. That water may be stable at 70 degrees but could phase separate at 30 degrees.
Years ago before ethanol people would leave their vented gas cans out and would either condense water from temperature air exchange or just flat get rain in them. Now with ethanol the alcohol makes it it's mission to draw water from the air every change it gets being it from open vent container or other means.