Final answer:
Aven and Connor Bradley suspected that the Cavanaughs were missing because they observed the rustlers riding into a canyon and disappearing.
Step-by-step explanation:
Aven and Connor Bradley suspected that the Cavanaughs were missing at Stagecoach Pass because they had observed the rustlers riding into a canyon and disappearing.
They also noticed that the cattle tracks all pointed west, suggesting that the red herd had been driven out of the canyon across the oval. This led them to suspect that Oldring's den, where the rustlers were hiding, was located in that canyon.