As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
UPON ENGINE START AT GAT IN IAD / FLT CREW RECEIVED LAV SMOKE INDICATION AND REPORTED A SMALL AMOUNT OF SMOKE. CAPTAIN ELECTED TO DEPLANE PASSANGERS THROUGH THE MAIN CABIN DOOR. ALL PASSENGERS AND CREW DEPLANED SAFELY AND THE FIRE DEPT WAS CALLED BY OPS. MX RAN AIRCRAFT COULD NOT DUPLICATE, CHANGED THE SMOKE DETECTOR.