No Sarx this never sounded like a runaway to me. I think ruling that out had to happen but at the same time LE was chasing down information about could Hailey have agreed to meet with someone she met somewhere via internet, traveling for sports etc., and then been abducted? She certainly would not have left money, shoes, her fav things in her room I personally do not believe, had she decided to run away.
I was thinking more along the lines of meeting an older boy at a sports meet, or even a spectator who she might have shared personal information with. Thus the need to check her belongings, books, notes, to see if she left any clues.
The fact however that they searched the entire school, and other student lockers tells me they already knew that something was seriously wrong. They were looking for any indication that Hailey had been abducted or harmed already IMO.