A CRJ-900 FLT CREW EXPERIENCED MASTER WARNING 'SMOKE TOILET' SHORTLY AFTER TAKEOFF. THEY DECLARED AN EMERGENCY AND RETURNED TO LAND AT THE DEPARTURE AIRPORT.

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APPROX 30 SECONDS AFTER TKOF (I WAS PF) THE MASTER WARNING SOUNDED AND THE CAPT READ OUT LOUD 'SMOKE TOILET' FROM THE EICAS SCREEN. HE IMMEDIATELY SAID 'WE'RE HEADIN BACK' AND I AGREED. WE CONTINUED THROUGH THE (NORMAL) AFTER TKOF CHKLIST; THEN NOTIFIED ATC OF OUR SMOKE INDICATION AND THAT WE ARE DECLARING AN EMER AND WOULD LIKE TO CONTINUE STRAIGHT OUT. ATC GAVE US A HDG AND SAID THEY UNDERSTOOD WE WANTED TO COME BACK TO LAND IN ZZZ AND THAT THEY WOULD HAVE THE TRUCKS WAITING FOR US ON THE RWY. I THEN CALLED FOR A DSCNT CHKLIST WHEN ABLE FROM THE CAPT. THE CAPT TOLD ME TO TAKE THE RADIOS WHILE HE MADE CONTACT WITH THE FLT ATTENDANT. THE FLT ATTENDANT INFORMED US THAT THERE WAS A LOUD HORN COMING FROM THE REAR OF THE CABIN; BUT THAT SHE COULD NEITHER SMELL NOR SEE SMOKE COMING FROM THE LAVATORY. HE INFORMED HER THAT WE WOULD BE RETURNING BACK TO ZZZ AND THAT IF SHE NOTICED ANYTHING AT ALL TO CALL US AND LET US KNOW IN THE FLT DECK. AFTER RECEIVING A DOWNWIND VECTOR FOR RWY 30L; THE EICAS ANNUNCIATION DISAPPEARED. WE STILL RAN THROUGH THE DSCNT CHKLIST AND THEN I CALLED FOR THE EMER CHKLIST FOR SMOKE TOILET. THE CHKLIST CALLED FOR US TO PREPARE TO LAND IMMEDIATELY AT THE NEAREST SUITABLE ARPT. DUE TO THE LACK OF TIME AVAILABLE; AND FOR ALL PRACTICAL PURPOSES; WE CONTINUED WITH SETTING UP THE APCH AND RUNNING OUR NORMAL CHKLISTS; INSTEAD OF SEARCHING OF THE 5 CIRCUIT BREAKERS THAT THE CHKLIST CALLED FOR US TO OPEN. THE LNDG WAS UNEVENTFUL. WE TAXIED OFF RWY AND SWITCHED TO A FREQ TO TALK WITH CFR. THEY AFFIRMED THERE WERE NO ABNORMAL HEAT READINGS COMING FROM THE TAIL OF THE ACFT. WE DECIDED TO TAXI BACK TO THE GATE AND HAVE THEM FOLLOW US TO DEPLANE THE PAX.