When are mystery parts available for world events?

For some world events, there are sometimes "mystery parts" which can be valuable parts of whatever the reward is when delivered. Under what circumstances are these available?

They are not common in my experience, and it seems that they are only present when the overall reward item is a complete airplane, rather than just a plane part and not in the case when the reward is just bux. When there are mystery parts available, it seems worth keeping an eye out for them far beyond the minimal (usually 50) job participation in the global event.