"When the images were discovered, the airline should have checked the entire jet, not just the auxiliary power unit, according to the complaint. The unit is a small turbine that drives a generator mainly for power on the ground.
With the 747 in a secured area of the airport and the graffiti on the tail about 30 feet off the ground, the images should have triggered a more-comprehensive reaction, according to the complaint. A pilot’s suggestion to the crew that images were applied when the plane was in South Korea before arriving in San Francisco should have raised alarms about safety in that country, the attendants said."
United Attendants Say 13 Fired for Protesting ?Menacing? Jet - Bloomberg