He moonlights as a sherriff at Harper's Island.
It all makes sense. Bobby lost his wife. Charlie Mills lost his wife...
So what if we never knew he had a daughter. Would you want your daughter near the Winchester brothers? I think not.
So, Bobby goes to Harper Island to investigate murders. As meg_tdj points out, everyone keeps saying the killer is dead. Maybe he is. Who can pass over a litle ghost hunting on the side?
The boys would never even figure it out. They come across Bobby's uniform and his badge and it's no big deal. Bobby scams all the time. That could just be part of his stash.
And seriously? I bet he's really suspicious why someone who looks like Ruby and someone who looks like Ron are walking around. At a place where people are dying, no less. These are the times of the apocalypse, now. One can't be too careful with all these seals and demons and witnesses walking around...
I guess we'll just see if Bobby makes it out alive. And if he doesn't? Well, he's good at living double lives. i'm sure he'll pop up again somewhere. ;)