Você está certo. O script indica que ele se transformou (depois de sangrar até a morte), então subsequentemente escreveu a mensagem.
KENNETH: What a minute. I see him. He's on the roof. I think he's writing something.
[POV BINOCULARS]
We see Andy, his back to us, working on a dry erase board. He turns and holds it up. The board is smeared with blood. His eyes dead. Andy has turned.[ON THE ROOF]
MICHAEL: What does the sign say?KENNETH: Tell her to get out of there, now.