A B757-200's aft lavatory smoke detectors sounded during preflight and again on descent to the destination. The APU was initially suspected and was MEL'ed before departure. At the destination; the aircraft was removed from service and maintenance ferried because of the second smoke alarm in flight.

Narrative

Upon arrival at aircraft a musty smell was noted. Flight attendants complained that the smell increased and the 2 left lav smoke detector sounded during boarding. Maintenance was immediately contacted. Subsequently the APU auto shut down. Maintenance personnel determined that the source of the oder was the APU. The APU was MEL'ed. Both engines were started at the gate and the packs were run for five minutes with Maintenance in attendance. No further odor was detected and our flight departed forty five minutes late. Approximately two hours into the flight the 2 left lav smoke alarm sounded. The flight attendants were unable to silence the alarm; so I left the cockpit to investigate. I detected no smoke or odor. I also did not know how to silence the alarm. A passenger offered a large band aide to cover the speaker. The alarm subsequently stopped. Our flight continued on to the destination and during decent the flight attendants reported a return of the odor. I did not detect it in the cockpit. Upon arrival the flight attendants informed us that they would no longer fly on our aircraft. The return flight was canceled and we maintenance ferried it to a maintenance base. During our ferry flight the 2 left lav smoke alarm sounded again with no smoke or odor detected. On arrival the aircraft was turned over to Maintenance at the hanger for further investigation.