Of course, the fact that he’s the only survivor of the train wreck, with not so much as a scratch on him, is another clue that David is not an ordinary man.
As stated by Elijah in the film, David may have chosen the occupation as a security guard because he has a knack for protecting people – just as any super hero does.
In the scene just before David and Audrey go on a date, Joseph is playing with a GREEN and YELLOW action figure superhero. Since those seem to be David’s “colors,” this is another allusion to Daivd being a real life superhero.
Elijah mentions to David over the answering machine that water is like David’s Kryptonite. This is one of the first real clues that links him to his eventual super hero revelation, and more specifically, his parallels with Superman. Most superheroes have one significant weekness, and David’s is water. But as we all know, he even overcomes that obstacle.
Upon arriving home after his battle with the man in the orange jumpsuit, David hangs his security guard “uniform” up in his closet just as any other super hero would do, such as Superman or Batman.
After showing his son the newspaper article, Dunne motions to his son with a “shhh” to keep his identity a secret from Audrey. As in most comic books, the superhero tries to keep his/her identity a secret.
At one point, Audrey asks David if he’s knowingly keeping her and their son at a distance. This is something that superheros tend to do, to keep their families safe.