As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
SMOKE ALARM WENT OFF AFTER TAKEOFF. CREW DECLARED EMER & RETURNED, LANDED DUE TO SMOKE ALARM SOUNDING. FOUND NR 2 SMOKE DETECTOR IN MAIN CABIN TO HAVE HAD THE BEACON BULB FAILED. INSTALLED SERVICEABLE BEACON BULB. OPERATIONAL CHECKED GOOD IAW CMM PAGE 6.