The very kind and competent person on the other end of the line said, "It's not in your model year's handbook, but in subsequent years there's an explanation of how to prevent this. When an airplane — even one using avgas — flies at altitudes where it's very cold, the water that is regularly dissolved in the fuel precipitates out in the form of ice crystals, which then can block your fuel system. Then, as your fuel system warms up, the ice melts and the water dissolves back into the fuel. When you drain warm fuel from the sumps, you won't find water, because it is dissolved in the fuel. The solution is to use a fuel-system icing inhibitor like the product called Prist, or isopropyl alcohol."