This, or something like this. I think the tip, if there was one as you state, was likely anonymous (but for some reason very credible). The PC was directed in part at this person, hoping their conscience would crack and they would come forward (LE begged for them to trust). I do think the vehicle increased in significance after reviewing their initial interviews. I don't think they forgot about it, so much as dismissed it or cleared the owner(s) based on a false alibi. That's why they need information about the driver of the vehicle, in order to place their POI at or near the scene of the crime in the time frame of the crime to unravel their alibi.