Terror In The Skies!!! Australian Style

Lots of people defended Annie Jacobsen's bizarre episode of anti-Arab paranoia by pointing out that the plane's crew was apparently concerned as well. Jacobsen has cited support from multiple airline employees who have written in to tell her that Arab terrorists are constantly probing airline security and conducting "dry runs" of attacks. Surely these reports are trustworthy. Surely air travel professionals would have calm and reasoned responses to innocuous, terror-free in-flight events.

Surely not.

Ninety minutes after taking off from Sydney Airport, a flight attendant on a United Airlines flight bound for Los Angeles found an airsickness bag - presumably unused - in a lavatory with the letters "BOB" written on it.

The flight attendant decided that the letters stood for "Bomb On Board" and immediately alerted the captain, who decided the risk was serious enough to turn the plane around and land back in Sydney. [...]

Why in the world would someone decide that out of all the possible meanings that "BOB" scribbled on an airsickness bag could have, its presence on this particular airsickness bag on this particular flight must mean "Bomb On Board"?