After poking around the vents today, doing more work to insulate and clean
up before winter I noticed that the vent stack on my furnace is for the most
part coated in a white fine powder material, as well as rust flakes. I
understand from reading about CO, that the white powder indicates CO
problems.

But what does that mean, does it mean that the vent is leaking CO, or does
it indicate the furnace is venting excessive CO (maybe burning inefficiently
due to age)? The powder is only forming on the joints between pipes, which
to my mind says exhaust gas leakage.

Just for reference my furnace is 23 years old, and none of the venting
joints are sealed.