Unknown rectangle with no lights passed through the photo frame of a camera set up to photograph comet Neowise.

While out photographing comet Neowise Tuesday evening from my front yard I spotted an object that flew through the field of view I had composed with a 450mm lens (an arc of just under five degrees).  The unknown rectangle had no lights of any kind and had a dull, tan coloration.  Though late, I suspect I could see its color due to sky glow from the city of Everett 10 miles to the NW.  The object entered the photo frame from the lower left and exited at the upper right.  Its straight line trajectory took it above the image of the comet.  The size of the comet comprised about 1/20th of the photo frame.  The object was 2 or 3 times larger than the comet’s nucleus.  The object’s rectangular shape seemed a bit broken up along the top with black lines (shadows?) in the middle.
 The camera gear was a professional mirrorless camera with a long telephoto lens on a tripod.  With exposure set for the comet I did not get a photo of the object.
Witness is a long-time photography teacher at Seattle- area colleges.