What was found
(CAN) HEAVY SMOKE NOTED COMING INTO AIRCRAFT AFTER TAKEOFF. CREW DIVERTED BACK TO DEPARTURE AIRPORT, DELCARED AN EMERGENCY AND LANDED WITHOUT ISSUES. 1 PAX ON-BOARD. MX IN WORK, PERFORMED INSP AND FOUND DE-ICE FLUID ON TAIL SECTION STILL DRIPPING DOWN AND AROUND APU INLET. DE-BRIEFED CREW AND THEY DID HAVE APU RUNNING AT TIME OF SMOKE. ENGINES & APU RUNS CARRIED OUT, DE-ICING FLUID WAS BURNED OFF AND AIRCRAFT WAS RETURNED TO SERVICE. (TC# 20180314006).