That seemed like a pretty dumb decision. If the killer, or killers, came back to the store, what was one person suppose to do with a pistol that couldn't be done with a rifle? It was totally unsafe for him to stay there.
someone remains behind at crime scenes till the investigating crew arrives, mostly to make sure no one else comes along and disturbs anything. Its very normal. At that time, I don't believe that they were expecting the criminal to return to the scene, after all, no one had returned to the trailer site.
But the steaming coffee pot implied the monster had struck shortly before, within probably less than 30 minutes. It would boil off after that time. Of course I suppose they weren't expecting monsters.
"Extremism in the pursuit of moderation is no vice."