This is the second time I've seen this happen while rebuilding a carburetor.  The pie plate contained fresh fuel when after no more than 5 minutes, it developed a milky appearance. Temperature is almost 90* with 70% relative humidity, one of those hot, humid sticky days in south east Michigan. Evaporaation kept the fuel cool while it pulled moisture like a sponge out of the atmospherte.  The first time I noticed beads of water rapidly forming71% of original size (was 707x19) - Click to enlarge on the bottom of the pie plate. 

64F100 57FAIRLANE500

Yep, I've taken apart carburetors with water damage from sitting with fuel (white corrosion)