What was found
SMOKE WAS OBSERVED IN THE CABIN AFTER START OF TAXI TO ACTIVE RUNWAY. THE CREW SHUT DOWN ENGINES, SHUTOFF ALL POWER AND DISCONNECTED BATTERIES. FOUND SOURCE OF SMOKE TO BE FROM A NO SMOKING/SEATBELT SIGN LOCATED IN THE LT SIDE OF CABIN AT FRAME 20. THE AIRCRAFT WAS SECURED AND FERRIED TO A MAINTENANCE BASE AND UPON FURTHER INVESTIGATION WE FOUND NO CONCLUSIVE EVIDENCE BUT SUSPECT THAT THE PROBLEM ORIGINATED AT THE INTERFACE OF THE WIRING TO THE NO SMOKING/SEATBELT SIGN.