What was found
FLT 4063 - CREW NOTICED SMOKE IN AIRCRAFT DURING TAXI. THERE WERE NO ASSOCIATED PACK, DUCT, OR BLEED INDICATION WARNING LIGHTS ILLUMINATED. MAINTENANCE RAN BOTH NR 1 AND NR 2 ENGINES FOR 15 MINUTES, CYCLED BOTH PACKS, AND SHUT BLEEDS ON AND OFF. MAINTENANCE ALSO INSPECTED ELECTRIC BAY, ACM HOSE AND ENGINE BLEEDS FOR OIL THEN RAN ENGINES NR 1 AND NR 2 FOR 15 MINUTES AGAIN. MAINTENANCE COULD NOT DUPLICATE DISCREPANCY. AIRCRAFT RETURNED TO SERVICE. (X)