After midnight I was on my way home from work, I was driving from the Fairwood neighborhood in Renton, WA. I was driving down Benson Rd. where there is a good birds eye view of the Renton Valley and the City of Seattle.

While driving downhill, I suddenly saw flash of light in the sky out of nowhere.  It was extremely bright and it seemed to be very vast.  It startled me, as I instantly thought that only a nuclear bomb could cause such a flash over the city.  I can only explain the flash as it being like a giant LED flashlight, big enough to light up the entire sky.  It was like the giant flashlight, flicked on and off in a split second.  
Being curious as to what this flash was, I continued looking up vertically while trying to safely drive.  I could see airplanes in the air and a big full moon, I'm sure it wasn't lightning... It wasn't overcast nor was there any rain at all and  the flash was silent....  No thunder.  It definitely didn't look like lighting or natural phenomenon I've ever seen.
I went home and put on and browsed all the national and local cable news stations.  I'm sure someone else saw this.  I thought that there might be an explanation, but couldn't find any information.  A couple hours later at about 3:40 am I went back out to my car to grab something before bed, and again, I saw another big bright quiet flash.  I'm sure a surveillance camera caught this somewhere or that someone else in the area I live saw this.
I will be checking back to see if anyone else has a post on this subject.  This an experience I will never forget.