[jayastout]

Yes, that's a good point; it's certainly possible that they were mistaken. But this was from the group's official mission summary report and I wouldn't think that they (he?) would go out of their way to debrief the point unless they were certain of what they saw. As for it being exhaust, or a leak of some sort (as I have also considered and others have suggested)...it seems odd that all three aircraft would have the same leak. And certainly the characteristics of the vapor would have changed as the aircraft maneuvered, so that it would be difficult to mistake it for a streamer.

Perhaps it will just remain a mystery.