An access panel / faring. Not too dangerous to THAT plane. It is a hazard to the plane that takes off behind it that sucks one in to the engine and damages some of the blades. (unlikely but possible) A piece of “FOD” (foreign objects and debris) took out the Concorde in France.
If no had had spotted it in the cabin they would have just covered it with some ‘speed tape’ and got it at the next stop.