The meta answer is that I didn't think of the ring thing until working with Matthew on
#3 , because we needed some clue for the PCs to find.
I'd say that Macbannin could have had one, and you can probably just hand-wave that the PCs didn't notice it. He's a rich guy; he had a lot of jewelry, and the ring isn't magical, so I figure most PCs will gloss over it.
Or maybe he didn't wear it because, again, he's a rich guy, and the ring wasn't fancy enough.
Or maybe he didn't wear it because he knew Saxby already knew he was involved in this secret project. Now, Macbannin didn't think he was working for an international conspiracy called the Obscurati. He just thought he was working for his homeland.
Creed too might have had one, but since he's in disguise a lot he might have had a collection of rings for different costumes.
Saxby, ring? I'd say that Macbannin probably didn't give her one. Macbannin probably got his from Governor Stanfield, who also might have included a handful of other rings with a suggestion, "Give these to your people who might get into trouble." He would have figured that if Saxby got into trouble, the only higher authorities would be people who already knew what was going on. And giving her a ring could possibly be a liability, so no ring.