As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
AFT LAV SMOKE DETECTOR WENT OFF CLIMBING OUT OF DEN. SMOKE HAZE IN CABIN, RETURNED TO DENVER. *S/D* APU HAD MASSIVE COOLING FAN FAILURE, METAL IN THE SYSTEM. OIL WAS SUCKED INTO THE APU INLET PLENUM THROUGH THE COOLING FAN DUCT CAUSING THE SMOKE IN THE CABIN. THE APU WILL BE DISASSEMBLED FOR OVERHAUL.