Boomerang craft, silent, no lights, walking speed

Was watching Mars in the SE sky that night from recommendations from the Star Hustler show on PBS earlier that week.  The sky was clear with no clouds or moon.  Out of the corner of my left eye I saw motion in the sky.  It was a boomerang (chevron?) shape with no running lights or sound, moving at "walking speed" silently from N to S.  It seemed a similar shade of dark as the sky, blotting out the stars as it passed, and since there were no lights it gave only the slightest hint of some kind of detail in the center.  It covered a good portion of the sky, (I'll use the ever popular "Size of a football field" description). The people with me marveled at it a few moments but seemed to pay it no further attention. I intended to run in and call the FAA control center in Longmont where it seemed to be headed but was locked in place watching it till it faded in the distance to the south.  I was met with the usual ridicule from family and friends.
Date is approximate.  PD