I doubt she would have disposed of him any where nearby. Too risky. And wherever her phone pinged, that might've just been a stop along the way to where she was actually headed (she probably left the phone somewhere and then continued on).
What wilderness or remote places has she been to around there before? Did she camp, hike, hunt, or fish? Places that are within say 1 1/2 hr drive of the house? Mountains would be best, and might avoid the need to dig a grave. Wouldn't be far from the road. But she would've had to find a place with no witnesses, so not any scenic or popular routes. Heavily wooded areas a possibility, but not as likely IMO..
Might be video cams on some of the routes up into the mountains or out into remote areas though. And I assume they've checked GPS /on star on the truck (maybe they're awaiting results), and gas station cc receipts.
Perhaps they can also test the soil on the truck tires, her shoes, and on any implements in the bag. They could also test those same items for Gannon's DNA. Also, check the house for antifreeze (incl trace amounts on glasses). But they've probably done all that, and again, it's just more circumstantial evidence without finding Gannon. Wish we could bring him home