IMO it is possible and no matter where the DNA was recovered from, the defense will probably try to argue it away. (I'm not saying that the DNA does not belong to BG, just that I'm sure it will come up at some point if there's a trial. It usually does.) This is one of the reasons I'm glad that they called off the search on the night of the 13th. It lessened the likelihood of someone accidentally contaminating the crime scene.