Reporting him missing could have been a strategy employed by the attorney out of guilt, or even the parents whose conscience may be starting to bother them. Once he's reported missing, this opens the door for law enforcement to start looking for evidence to locate Brian without a warrant (social media, phone records, etc) and without the parents having to state they think he's guilty or indicating they knew he was guilty from the beginning. It basically opens the door for them to cooperate without openly saying they support the criminal side of the investigation, then to change their stance and be supportive once something is "found", if that makes any sense.
While the legality of using evidence found during the missing person search may or may not be fully admissible in Gabby's case, accessing Brian's phone and social media could give them a lot of insight about Gabby's whereabouts or where to look for more evidence.
MOO OMO or whatever we're currently using to indicate speculation