My suspicion--not necessarily related to the car, but in reference to having previously interviewed BG--is that someone alerted LE to BG, but did so with a random SM-driven or malicious tip. In other words, someone fingered the right guy for the wrong reason.
If that's the case, then when LE interviewed the guy, they probably didn't go about it too thoroughly because it resembled on the surface all of the sketchy BS tips they were receiving. I'd guess that this tip came in at least a few months after the murders--after the release of the OBG sketch, most likely around the time that DN was being investigated.