What was found
ACFT DID AN AIR TURNBACK INTO BTV FOR SMOKE TOILET CAUTION MESSAGE AND THE LAV SMOKE DETECTOR WAS GOING OFF. EMERGENCY WAS DECLARED. UPON ARRIVAL BACK TO BTV THE FIRE DEPT SAID NO EVIDENCE OF SMOKE WAS DETECTED OUTSIDE THE ACFT, AND THEY USED A SMOKE SNIFFER INSIDE THE ACFT AND NO FUMES WERE DETECTED. THE F/A DID SAY SHE SMELLED A BURNT PLASTIC SMELL. RT ENGINE NUMBER 3 CARBON SEAL SOURCE OF OIL WETTING ON COMPRESSOR. REPLACED RT ENG FOR SMELL IN CABIN. R&R RT ENG IAW AMM 71-00-00 & CHECKLIST. OP/C,LK/C GOOD VERIFIED PART EFFECTIVITY. PERFORMED ALL BREAK-IN RUNS & OP/C LK/C IAW AMM 71-00-00. OP/C LK/C GOOD. 0 OZ OIL USED POST RUNS.