If local, it would suffice for him to join the group early in the morning...
Sometimes I think, since the crime scene looked "strange", per R.I., what if it was messy, with lots of random stuff there? If the killer worked at a local knickknacks store, or a popcorn stand in a movie theater, something like that...a place where lots of people leave their DNA, and get his DNA with the things they buy. A library, for example. He can always explain his DNA being found in many places.
If BG is a random serial killer, then his DNA might be unknown, but if he is of a regular Anglo-German stock, I can't imagine they can't find his cousins. These days, a person with totally unknown and unmatched DNA would virtually have to be an alien.
Less likely than someone whose DNA is "legally" in the mix.
P.S. edit to add: a local doctor's or a vet's office is the place where there is a lot of DNA transfer.