Oddly enough, it was the silence that had woken Bobby. Sure, there had been patches of it over the past ... hell, it had been a few days hadn't it? But this one had gone on longer than the rest, and it was enough to trigger his paranoia. All good hunters had a finely-tuned sense of paranoia. Not the kind that latched on to every little thing insisting that it was a sign of impending doom, but something quiet, almost domesticated, quietly and calmly pointing out when something just wasn't right.
He'd fallen asleep at his desk again, which wasn't all that uncommon an occurrence, especially these days. He feigned sleep still, it was something he was good at, just another ticky-box on his list of skills, but he was listening closely and it didn't take him long to hear the footsteps, someone trying, and managing to be quiet. As soon as the front door closed, Bobby was on his feet, retrieving his shotgun from where it was propped next to the library door, moving just as quietly as his quarry had, no sense waking Dean up for this, it would only lead to another fight and he wasn't ready for that just yet.
They'd already had it out with each other earlier over just what it was that was, probably, killing Sam, and while the detox had been Bobby's idea in the first place, it wasn't working which meant that it was time to change tactics, but Dean wasn't ready for that, so they'd agreed to disagree and give it a few more hours.
Bobby eased around the side of the house and out into the yard, Sam wasn't hard to spot, really, the only other living thing out there at that time of night. The shotgun was just a precaution at the moment, held in the crook of his arm, "Sammy." It was a question as much as it was a warning.