As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
CREW REPORTED THAT ON TAXI OUT THE LAV SMOKE INDICATION CAME ON EICAS AND FLIGHT ATTENDANT REPORTED SMOKE IN AFT CABIN. MAINTENANCE INSPECTED SYSTEM AND REMOVED AND REPLACED THE APU. ALL OPERATIONAL CHECKS WERE GOOD. AIRCRAFT RETURNED TO SERVICE.