As the town's resident troublemaker, Dallas naturally got called down to the police station once the police discovered Bob. Luckily, he was at Buck's house that night, so his alibi checked out.
When the police asked Dally about Johnny and Soda's whereabouts, he told them that they were headed for Texas. Five days after the incident with Bob, Dally comes to visit the boys at the abandoned church in Windrixville, he tells them:
"The fuzz won't e lookin' for you around here. They think you've lit out for texas."
Seeing as how the police weren't looking for them in Windrixville, they figured it was okay that they leave their hideout at the church and go out for a real meal at the Dairy Queen.