No, pressure washing then engine should not have effected the
system. It was the "heavy rain" that got me thinking, but since you checked and there was no trace of moisture near the pump
That probably wasn't it either. May have just been an intermittant
internal electrical glitch with the pump. When the LED comes on solid, there is a fault in the electrical portion of the pump.
Only time will tell. If it happens again, have your MB retail agent
check for computer fault codes stored in the system.
I hope all is well from here on.