In the Inventory and Return of Search Warrant that was posted earlier it says they performed a search of the car on the 11th and the 17th.
Speculation: I think that the initial search of the car was allowed by the parents.
This search was conducted on the 11th. I think that something was found in the car at that time that gave the police the impression that some type for foul play had taken place. It seems like the investigation took a turn at that point.
I am given the impression that the police had to apply for the search warrant for the one that was done on the 17th. This search was very thorough. That means that on the 17th they knew where the glass that was found came from. I assume it came from the CB house.
Maybe on the 11th they were still trying to find her and on the 17th they were processing the car for the murder.