Thanks one and all for solving the mystery - i.e. all likelihood is the first gong signals "landing gear up".
Which beings me to a related point: at time of crew release (second gong) the seat belts light flashes two or three times, and one can count on at one passenger to misinterpret this as the passenger release signal and head off to the toilet. Can't the flashing be suppressed?
Christep: your point that this should be in Fragrant Harbour forum is well noted, but contributions here are er um a bit less confrontational.
HKPAX