What was found
GEG - AIRCRAFT RETURNED TO FIELD DUE TO SMOKE IN THE CABIN. MAINTENANCE FOUND THE WATER SOCK DRAIN PLUGGED FOR THE CABIN PACK. THE DRAIN WAS UNPLUGGED AND THE WATER SOCK WAS CLEANED, IT WAS FULL OF WATER MIXED WITH OIL. SYSTEM WAS OPERATIONALLY TESTED AND A TEST FLIGHT ACCOMPLISHED, TESTS NORMAL. THE APU WAS SUSPECTED OF PUTTING THE OIL IN THE BLEED AIR SYSTEM AND IS CURRENTLY ON MEL.