Hmm. Well, according to a news article about the locating of the Elantra, it was two WSU campus police who, after receiving word early on (Day 2 or 3, just after the early video was looked at by Moscow PD) located all the Elantras at WSU (one of which was from PA and would not have had a front license plate and belonged to...a criminology student, which must have caught their eye).
The white sedan cruised past the gray, three-story rental home on a dead-end street in Moscow, Idaho. Then again. And again.
komonews.com
And the article continues:
At that point, they did have BK's own Elantra on their radar. I think as the video continued to come in, they were more and more sure it was BK's ELantra, registered to his name at WSU.
Of course they needed the genetic genealogy as well (whatever sort they used), but I now understand the twinkle in Chief Fry's eye when their suspect grad student appeared as a match.
The fact that this criminology grad student (yesterday's article by the criminology prof explains why) had just changed plates must have made him suspicious. At any rate I believe they had him under surveillance since about Nov 29.
These are events that were unfolding in real time, isolating BK as a promising POI, so they already had him on their radar before he left town.