Big rectangle drone looking object sat in midair about 100 yards away for about 10 minutes then flew up and away into the clouds.

It looked like a drone out our window 100 yards or so.  Just sitting in mid air maybe taking photos or video.  But if it was, it was the biggest drone I have ever seen.  Plus it was not shaped like a drone.  Looked more like a big rectangle.
Later I thought it might be a balloon until I looked at my photos.
It was sitting out the window for about 10 minutes then just slowly started going up and away from my window.  I watched it mesmerized for quite some time thinking it was going to fly back to its owner after while.  It didn't.  I just kept flying or floating upward.
Me and two of my co-workers just watched it for a while thinking it had to be some cool new drone.
After while as I started to process what I was seeing I thought it would zip off at the speed-of-sound or something but it didn't.  It just kept going up and away until it was out of sight.