MOO
Thinking about a probable route, I think it is likely to have been anger resulting in her violent attack, smothered him trying to silence him or he was killed or by exposure running away from her. All F1s.
Scenario I would use to search.
They are going out in the truck to Play it Again Sports on N. Academy NW of CS.
They pass the Army Reserve Center on Marksheffel. Gannon seeing the center is reminded how much he misses his Dad distraught cries or screams for his Dad, maybe ia incontinent violence follows, he dies right then or runs into the Jimmy Creek Wash north of thebReserve Center.
LS is stopped roadside and there she is with his body outside the truck, or with him running into the creek wash.
She needs to think, gets into truck continues automatically toward PIAS, north on Marksheffel turns west at Constitution. Stops at the Big Wave car wash on Tuft or to the Phillips 66 on Constitution.
That Phillips 66 is next to Cimmaron Hills Vapor Source, a a Wells Fargo, a Subway and UC Health ER. All look to me like their cameras or intenet Wifi linking could come up as evidence.
Assuming this happened, she then planned how to get out of her crime, call him a runaway...after all he WAS running away from her one way or another.
Due to being worried about visible bruising she thinks best if he is found after too late to detect that.
She takes a complicated road trip, worries, cleans calls the police about Gannon not returning. Next day she gets the rental, which in a way is admitting that he would not come to her even if with she was with his Dad.
In thebrental before picking up AS at CS Airport she hides him better in the Jimmy Creek, or takes him somewhere else, or does nothing but continue to be driving around to complicate the incident.