So in the Prequel, they have a couple of shots focused on the security guard in the Oval Office.
You never really see where he’s standing in relationship to the President.
And Nixon insists that there are no monsters in the Oval Office, which means he can’t see anything unless he looks behind him.
The monster at the end isn’t behind him, though, it’s next to him, and it’s wearing an outfit similar to the guard: dark suit & tie.
What I’m thinking is that the security guard is actually the monster/Silence we see at the end, and that one of the powers that the Silence has is the ability to project a different appearance, like the Calvierris or Prisoner Zero.
Either that or the Silence can inhabit different creatures temporarily, and the rest of the time it’s intangible.