As any David Lynch fan will tell you, it really stands for “Beware of Bob”

From “Sick Bag Note Caused United Flight To Turn Back”, July 28, 2004:

…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.

One suggestion: aviation officials ought to have paid closer attention to the phrase “FIRE WALK WITH ME” that was scrawled on the bag’s flipside.