Ohh, you beat me to the punch.
Well, heres one last thing I found.
http://medicluigi.tumblr.com/post/29...eavy-and-scout
Notice how the Spy, Heavy and Scout are all looking at the Engineer? Those are the 3 classes that were in the Secret Rooms in Foundry and Doomsday. They are all staring at the only man who could have created the Robots. And the Robot Heads in the Engie Tease video help too.
And the tanks that mysteriously appeared are Blue, so they can't be owned by Gray. The Blu Engie, perhaps?
So...could the Blue Engineer be the one who made the Robots?