What was found
ON TAXI, SMOKE LIGHT ILLUMINATED ON MASTER WARNING PANEL, SMOKE SENSOR ON FLIGHT ENGINEER PANEL ILLUMINATED IN NR 6 & 7. APPROPRIATE EMERGENCY CHECK LIST ACCOMPLISHED. RETURN TO BLOCKS. ON RETURN TO BLOCK, FOUND DURING DUAL PACK OPERATION OF WATER VAPOR EXITING FROM OVERHEAD DUCTING. OPERATED PACKS IN HEAT MODE TO CLEAR WATER VAPOR. THEN COOLED CABIN, AND PERFORMED MAIN DECK SMOKE DETECTION ADJUST.