What was found
(CAN) ON CLIMB OUT, SMOKE NOTED IN CABIN. APU FAULT AND APU OIL PRESS CAUTION MESSAGES POSTED. AN EMERGENCY WAS DECLARED AND THE AIRCRAFT RETURNED TO WHERE AN UNEVENTFUL LANDING WAS PERFORMED. MAINTENANCE FOUND AN APU EXTERNAL OIL LEAK - EXACT LOCATION NOT KNOWN AT THIS STAGE. THE APU WAS REPLACED. AIR CYCLE MACHINE, PRECOOLER, HEAT EXCHANGER, WATER SEPARATOR AND DUCTS HAE BEEN REMOVED FROM BOTH PACKS, CLEANED AND REFITTED. AIRCRAFT BACK IN SERVICE.