It was an object between two clouds, perfectly square. It was a brighter white than the whitest clouds.

I was driving (I believe toward Seattle, but I am not sure which direction I was driving), and I saw a perfectly square object between two clouds. 
I moved my head in all directions, thinking it might be a reflection in my windshield.  It remained as white and square as it first appeared.  I was so shocked that I failed to note the time, direction or even the date of the event. 
In seconds, it disappeared behind one of the clouds.  It almost looked as if the could moved in front of the object. 
The object was on the same level as my eyes, so I knew it was nothing close to the ground or the mountains.  I cannot recall whether it was over the mountains, or whether it was north.  My sense tells me that it was over the Olympic mountains, but I just am not sure. 
I was a legal secretary/wordprocessor for many years.  At present and for the last year, I am self-employed as a caregiver for senior citizens on Mercer Island, where I reside. 
I am very observant, and find myself "looking up" much of the time.  This is the first time I have actually seen a UFO.
Date is approximate.  PD