As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
BIL- WHEN ENTERING AIRCRAFT IT WAS FULL OF SMOKE AND THE LAVATORY SMOKE ALARM WAS GOING OFF. APU WAS RUNNING, BOTH DUCT TEMPS IN GREEN ARC. REPLACED APU. GROUND RAN APU, NO LEAKS NOTED. ALL INDICATION NORMAL DURING RUN. AIRCRAFT RETURNED TO SERVICE.