Symbolism, Imagery, Allegory
Early on in the novel, Ben tells Susan a ghost story. When he was a kid in 'Salem's Lot, he went into the Marsten House on a dare and walked up the stairs to the room where Marsten hanged himself. He opened the door, "And there was Hubie, hanging from the beam with his body silhouetted against the light from the window." As he watches, the hanged man opens his eyes (2.256-258).
In terms of plot, that scene doesn't have much to do with the rest of the novel. Marsten's ghost never shows up again, and the link between the ghost story and the vampires is never totally clear.
In terms of theme, though, the double opening—door, then eyes—is repeated in various forms throughout the book. There's Mike Ryerson in a trance reaching down to open Danny Glick's coffin and then seeing: "The eyes were open. Just as he had known they would be" (6.123). There's also Matt Burke listening to "the grind of wood against wood as the window was forced up" in the guest room (7.205). Then later, when Mike returns risen, Matt goes upstairs, pushes open the door, and lying on the bed, "Mike opened his eyes" (9.227).
Horror always gets a lot of mileage out of people opening doors and seeing the dark rooms that are behind them. In this novel, what's behind them, always, over and over, is death, whether in the form of Hubie, or Danny, or Mike Ryerson, or (at the end of the novel) Barlow. And that's what's behind the eyes opening, too—it's a soul where nobody's home. The opening of the door and of the eye is linked to the opening of the grave, which should be safely closed… but instead is swung wide for things to come out and pull you down.