The two Everett police detectives didn’t know the flag was missing until a stranger rang the doorbell at Fire Station 1 on Rucker Avenue. The man handed over a plastic bag to firefighters, along with a tale that led the detectives on the investigation of a lifetime. Nearly two years later — on Aug. 4 — a curator from the National September 11 Memorial Museum made a quiet visit to the Everett police station. She collected the American flag and its rigging rope and hardware. Detectives Jim Massingale and Mike Atwood breathed a little easier. They had kept their promises and secrets. The flag was safe. It was headed home.