As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
FLIGHT 3207 - BOS-BUF - ON CLIMBOUT, APPROX 300 FT, RECEIVED MASTER WARNING -LAV SMOKE-. RAN CHECKLIST PER QRH. RETURNED TO FIELD. FLIGHT ATTENDANT CHECKED AND CONFIRMED THERE WAS NO FIRE OR SMOKE. CLEANED DEBRIS FROM LAVATORY SMOKE DETECTOR. CREW PERFORMED ENGINE RUN WITH NO FAULTS. LAVATORY SMOKE DETECTOR CHECKED FINE.