Back to the Kobolds

After dealing with the dragon cult, the party can make their way back toward the crossroads and the trail leading to the kobold den beneath the volcano. They can get away from any stragglers without worry. The copper dragons will swoop down to eliminate any survivors that try to follow them. After the party is gone and back on their own journey, the copper dragons will descend into the valley to eradicate any remaining cultists. They will collapse the tunnels and destroy the fortress, avenging their betrayal and displaying the full extent of their rage with the cultists. Terrible noises will chase the characters on their way back south toward the dragon lair.
Unfortunately, this will also awaken the dragon to trouble stirring in the valley. As the party regains the area near the crossroads, they will be aware of an approaching patrol from the direction of the kobold lair. They can take shelter below the edge of the road among some low scrub trees, successfully staying out of sight with Stealth checks of DC10 or better. As long as half or more succeed, they kobolds pay no heed to the party.
If the party remains hidden, they overhear snippets of talk about "the restless dragon... at least we'll get a break... gone to deal with it, whatever is gone wrong... seems worried about something, given the increased patrols."
They barely pause as they continue past the intersection, down toward the swamp. After a few tense minutes, you hear nothing, and emerge to continue. They can proceed as normal, with a DC15 Perception check alerting them to one or two more patrols as they go, requiring a Stealth DC12 to get off the road and hide, again with half the party succeeding being enough to escape detection.
Before long, a terrible roar will part the night, as a wave of heat passes overhead toward the north. The party freeze in their place, realizing the dragon has just passed over, but it passes quickly given the talk they've overheard from the kobolds.
They can press on more quickly now, and if they choose, can get within a few miles of the kobold lair by morning light. They can find shelter away from the road before dawn, and though without a campfire, they can feel safe throughout much of the day.