Richard,

Having done it often, I can say it is far, far simpler than that. It
is obtained through the evanescent mode and optical couplers just like
RF couplers work on the exactly the same principle.

I'm told that this approach is relatively easy to detect in that it requires a
fair amount of mechanical motion to achieve. Single-mode fiber is often used
to detect minute changes in, e.g., bridges, and the idea is that -- while
there may be other causes of significant mechanical motion to a fiber --
detecting it in the case of a secure line is cause for investigation.

I've also been told, however, that you can use completely non-invasive
(mechanically) techniques to sniff fiber... something involving the
back-scatter of neutrons or somesuch. Don't know how much of that was
science-fiction vs. readily doable with the funding of a major government,
though.