As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
RETURNED T0 GATE. CABIN FILLED WITH SMOKE FROM AIR CONDITIONING SYSTEM. FOUND THAT APU HAD LOST ALL OIL IN TANK AND OIL IS VISIBLE IN APU EXHAUST. DIAGNOSTIC WAS PERFORMED TO CONFIRM REPORTED OIL LOSS. APU S/N 6391 TESTED OK. UNABLE TO ATTRIBUTE REPORTED OIL LOSS TO APU DEFECT. ACCOMPLISHED C-CHECK AND RETURNED APU TO SERVICE.