Bart Senior wrote

Source for next bit was Dutton's.

A ghost image is a false image moving in parallel
of a real object being tracked. They tend to be
more of an arc shape or fuzzy in shape compared
to the sharper image or a real object.

My interpretation based on my radar training.

They are formed by the signal taking a longer
path to the object. Most commonly from a reflection
off some other part of a larger vessel. This could
change the beam shape of the radar. Sometimes
you can get echos off thunderstorms.

This next bit was a joke. An optical coupler can't be shorted out
because it's is a card with holes cut in and and blocks
or passes light to a sensor.

Or perhaps your optical coupler has shorted out. grin

Joe wrote

If you see a ghost on radar are you seeing boobsie?

If not what are you seeing, and how would you define a ghost?