It seems as if the police are chasing cell tower pings. Given a suspect that is ignorant of this capability, it is effective. Given a suspect who has been a member of a fire department and no doubt involved in search and recovery, cell tower pings might not be so useful. A perpetrator who tosses the victim's dell phone into the back of some farmer's pickup can lead the police on a wild goose chase.
Wolfe is being charged with abduction, so police have not found evidence that he murdered them in Karissa's apartment.
The police have spent a lot of time searching Wolfe's home South of Gaston. He might have murdered them at his home.
My theory is that Wolfe murdered Karissa and William somewhere between Salem and Gaston, ditched Karissa's phone on some random vehicle around Hopewell / Amity, then went on to Gaston to either dump the bodies along the way or to dump them near Gaston.
One can hope that MW's vehicle has a GPS with route memory that can tell police where he has been. However; that still leaves a big search area.