HKPAX: 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?
Again, just as deduction from pax observation: The gong that's often used to release the crew sounds exactly like the gong when the seat belt sign is switched off or on. My guess is that that's exactly what the crew do for the crew release gong - they switch the seat belt sign off, then switch it on again.
Where that practice came from, I don't know, but again I would suspect that it's just one of those things that developed when it was discovered that there was no other easy way of giving the crew throughout the entire aircraft a quick and simple signal like that. So someone improvised, and it became standard practice.