If she was smart enough not to take her phone with her, or Gannon's, on that fateful Monday trip, it might be worthwhile to still check her phone and GPS on other vehicles in the weeks preceding Gannon's disappearance.
If premeditated it could very well be that she was scoping out locations prior to Monday.
Also re backing up the truck - I wonder if she wasn't really supposed to be driving that truck. She might have taken pains to make sure it was backed up just right so that AS wouldn't notice she took it.
ETA: If she wasn't supposed to drive the truck (purely speculation on my part), why did she take the truck rather than her own vehicle? I think it is significant that she took the truck. Where, or where did she go that she needed a truck?