The frames in FEC are idle frames between actual ethernet frames. So
you recall right, without FEC, you won't see this idle traffic.

It's very very good, because now you actually know before putting the
circuit in production, if the circuit works or not.

Lot of people have processes to ping from router-to-router for N time,
trying to determine circuit correctness before putting traffic on it,
which looks absolutely childish compared to FEC, both in terms of how
reliable the presumed outcome is and how long it takes to get to that
presumed outcome.