Probably about 95% of those cases--the parents know where the child is, but the child refuses to come home. Sometimes the kids are even going to school still--so in the better days (pre-budget cuts), the police could go to the schools and send them home. Not anymore. Your kid doesn't want to live at home and runs away? Noone's looking for them around here.
A lot of parents are afraid of being prosecuted for truancy (although in high school we prosecute students, not parents) so they report them as runaways, often times repeatedly. Kid comes home, 2 days lafter, runs away, 2 weeks later comes home again, runs aways again. Unfortunately, it's a very sad but common cycle.