What was found
(CAN) DURING CLIMBOUT, FLT CREW REPORTED SMOKE TOILET CAUTION MESSAGE. COMPLETED NON-NORMAL CHECKLIST. F/A REPORTED LAV SMOKE DETECTOR WENT OFF FOR A COUPLE OF MINUTES AND THERE WAS A MUSTY BURNING ODOR IN CABIN WITH NO SMOKE OR FIRE NOTICED IN LAV. ACFT RETURNED TO BASE AFTER CONFERRING WITH DISPATCH. SMOKE TOILET CAUTION WAS ONLY ON FOR 3 TO 4 MINUTES THEN EXTINGUISHED (BLEEDS OFF ENG IN TO). MX R & R THE LAV SMOKE DETECTOR IAW AMM 26-16. RAN BOTH PACKS AND SUPPLIED APU AND BOTH ENGINES FOR 30 MINUTES ON GROUND AND COULD NOT DUPLICATE MUSTY SCENT. OPS CHECKED THE SMOKE DETECTOR IAW AMM 26-61. NO FURTHER DEFECTS NOTED. ACFT RETURNED TO SERVICE.