I think this is by far the most important question that comes out of this last week's events (and the whole case).
Why would TS think that AS would readily buy that Gannon ran away? Did he buy that?
On Day 2, mid-morning, she says she rented a car so that AS and herself could go "door to door." At that point, there were no official searches, and neighbors were volunteering. It must have seemed high priority to do, if Gannon "ran away" because kids that age do not generally buy bus tickets or hitchhike. They go curl up in the bushes nearby or in the backyard of a friend's house or they are smuggled into a friend's house - for a night.
If AS did believe TS upon entering the rental car at the airport, he probably began to really ponder what was up (even before going to the neighbors). AS says he texts each of Gannon's friends (MSM, week 1) and I bet he also followed up with going to their houses.
Did TS want to pretend that the truck hadn't left the driveway? Would a perceptive person really buy that story?