ACR FLT CREW ON FERRY FLIGHT EXPERIENCES LAV SMOKE WARNING SHORTLY AFTER TKOF AND RETURNS TO DEPARTURE ARPT.

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WE WERE TO REPOSITION FOR REPAIRS AND A FLT ATTENDANT WANTING TO GO HOME. HE COULD NOT GET ON THE FLT THAT WAS LEAVING AT THE TIME DUE TO AN UNKNOWN REASON. HE WANTED TO COME WITH US ON THE REPOSITION FLT. THE CAPT APPROVED AND GOT DISPATCH TO ADD HIM TO THE RELEASE. WHEN WE WERE ON THE AIRPLANE MAINT CAME BACK WITH THE LOG AND TOLD US THAT IT IS TO BE A FERRY FLT DUE TO THE L ENG BEING 'MARGINAL.' THE CAPT LATER GOT THE SIGNED FERRY PERMIT AND WE LEFT. I DID NOT LOOK AT THE PERMIT AND I TRUSTED THAT THE PAPERWORK WAS IN ORDER BECAUSE I HAVE NEVER DONE A FERRY FLT BEFORE. COME TO FIND OUT LATER THAT THE FERRY PERMIT NEEDED TO HAVE THE FLT ATTENDANT'S NAME ON IT AS WELL. ALSO; AFTER TKOF; WE GOT A SMOKE TOILET CAUTION MESSAGE ON THE EICAS DISPLAY AND ASKED THE FLT ATTENDANT WHO WAS IN THE FIRST ROW TO GO BACK AND INVESTIGATE. THE FLT ATTENDANT CAME UP TO THE COCKPIT AND TOLD US THAT THERE WAS SMOKE IN THE CABIN. WE BOTH LOOKED BACK AND OBSERVED THE SMOKE. WE THEN TOLD ATC THAT WE HAD TO RETURN TO ZZZ TO LAND DUE TO SMOKE IN THE CABIN. WE WERE AT ABOUT 11000 FT MSL AND NO MORE THAN 20 MI FROM ZZZ. THE CAPT HAD THE FLT CTLS AND HANDLED THE RADIOS WHILE I PERFORMED THE ACTIONS IN THE POM EMER ACTIONS FOR THE SMOKE TOILET MESSAGE. I WAS ALSO RUNNING CHKLISTS AND SETTING UP FOR A VISUAL APCH BACK IN. WE LANDED WITHOUT INCIDENT BUT ONLY THEN DID WE FIND OUT THAT THE FLT ATTENDANT'S NAME HAD TO BE ON THE FERRY PERMIT.