Why am I getting this message when I try to save in FMB?

[major_setback]

It's been a long time since I used FMB, and that was with the FB series. i'm just starting to learn again. I made and saved a couple of simple missions and then decided to make one a bit more complicated.
I just spent an hour or two placing objects in a mission and when I tried to save it I got this warning message:

chief '4_chief' contains only one waypoint. Did you forget to set NPC flag?

I have only 2 aircraft in the mission, and each has about 5 waypoints. Neither aircraft (or call sign) is called '4_chief'. The mission is not called that either.

I have a few trucks on the ground with waypoints, but I can't see that they should cause a problem.

Any ideas as to why it won't fly or save? What does the warning mean?