What was found
RETURNED TO GATE AFTER SMOKE DETECTORS (BOTH AFT LAVS) WENT OFF. LEFT ENG RUNNING, LEFT PACK RUNNING FOR 5 MIN. APU AIR RUNNING, RIGHT PACK RUNNING SINCE PUSHBACK (20 MIN). LIGHT SMOKE (HAZE VISABLE IN REAR OF CABIN). CLEANED APU INLET BOX, PERFORMED DUCT BURNOUT.