As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
LAS - FLT 1204 - EN ROUTE LAS-DFW, SHORTLY AFTER APU START, DETECTED SMOKE IN CABIN. RETURNED LAS GATE WITHOUT FURTHER INCIDENT. RAN BOTH ENGINES. ACCOMPLISHED APU AND PACK BURNOUT PER THE MM. NO SMELL OR SMOKE NOTED. SYSTEM GROUND CHECKED NORMAL OPERATION. (M)