It probably just had some moisture in it since you had just washed it. Give them time to dry and make sure there is no dirt or anything on the lens/cover. If it still goes off with nothing around to trigger it then you probably have one bad.
This happened in my 01 and the way I checked them to find the bad one was to set the park brake and turn the ignition to the on position (do not start engine). Then put the shifter in reverse. Go to the back of the truck and put your ear against the sensors and you'll hear them clicking. The closer you get the faster they will click. The bad one will probably be silent. That's the one you need to replace. If they're all clicking the same then something else is wrong with the system. Could be a wire or the brain that controls it.
"3 can keep a secret if 2 are dead"
