Impossible Astronaut Hidden Clue?

Is there a hidden clue in The Impossible Astronaut? If you look carefully during the picnic scene after old Canton arrives, a shadowy figure can be seen lurking behind the shack.

It is visible in several shots leading up to when the future Doctor is killed.

After that, the figure disappears.

This could be a mistake, but cast your mind back to Flesh and Stone and the bit where the Doctor went back to Amy with his jacket on. Despite looking like a continuity error, it turned out to be a big plot point later on in The Big Bang. So we know it isn’t beyond Steven Moffat to put stuff in like this.

Assuming it’s not a mistake then, could this be the 909 year-old version of the Doctor? Or the Silent that Amy spotted earlier? Or someone else?

Thanks to CJ for sending this in.