A bomb threat that forced a United Airlines flight bound for Los Angeles to return to Sydney was caused by a note scrawled on a sick bag and was definitely a hoax, officials said on Wednesday.

United flight 840, carrying 246 passengers including a senior US diplomat, returned to Sydney about 90 minutes after take-off on Tuesday.

"I'm quite confident that after all the risk assessment that was conducted by United Airlines, New South Wales Police and the Australian Federal Police that this is in fact a hoax," police superintendent Peter O'Brien told reporters.

Transport officials have so far refused to confirm the contents of the note but ABC (Australian Broadcasting Corporation) radio said on Wednesday that an air sickness bag with the letters "B O B" scrawled on it had been found in a toilet on board.

The pilot decided the note could have meant "bomb on board" and returned to Sydney, dumping almost a full load of fuel before the Boeing 747-400 landed safely.

Several other possibilities were being investigated, including that the note could have been a popular flight crew acronym for a good looking passenger, or simply a man named Bob.

Flight attendants said crews sometimes use "BOB" to refer to "best on board", or the most attractive passengers on the plane.

"The innocent meaning of the acronym within cabin crew, and I guess even the general broader community at times, can mean 'best on board'," Michael Mijatov, from the Australian Flight Attendants Association told ABC radio.

"And simply it indicates from the perspective of people on board the aircraft, whether it's passengers or cabin crew, that someone sitting in a particular seat... to put it quite bluntly, looks good," he said.

The incident has also raised questions about how much information should be given to passengers in the event of bomb scares, after travelers were only told of an undisclosed threat to the aircraft.

"I understand that one of the areas where there may need to be some discourse is how much information is relayed to passengers in these circumstances," Australian Deputy Prime Minister and Transport Minister John Anderson said.

"There are other factors such as whether or not there might be people on board who simply might not be able to cope with the prospect that their lives may be in danger," he said.

Australian Federal Police said air crew and a number of passengers were interviewed following the incident. No charges had been laid.

All passengers had gone through security screening again and were to set off again for Los Angeles at 1 p.m. (0300 GMT).

Sydney Airport chief Max Moore-Wilton said an unidentified senior US diplomat had been on the flight. The diplomat was not US ambassador to Australia Tom Schieffer, he said.

(Reuters)