I can only speculate as to the meaning....but that's kind of what we are all here for!
JMO, (and I have gone back and forth on this like a yo-yo) the car could either belong to the witness who saw him there (his driver?, someone else entirely?), or it could have been driven by BG himself. I don't necessarily think it was there for the full 5 hours either. Again, this is JMO.
Perhaps BG got from the trail to another location very quickly and LE has been able to confirm this based on information that is available publicly. (This ties back to my opinion being that LE knows who it is and that this person is well-known.)
I don't think that LE is putting out any incorrect assumptions to the public.....they are too deeply invested in this case to go this route at this point in time. Especially now, since some people seem to be rather shaky in their faith that LE even knows what they are doing, based on the release of the second sketch which is a vast difference to the first sketch.
They are being very careful with every piece of information they release and, in my opinion, this is to protect the integrity of their investigation as well as to send messages directly to the monster on the bridge.
All JMO