What does this colour indicate about a surface?

Hi,

I have a model made up of many surfaces, but one is coloured differently to all the rest....

I've not applied any appearance and have tried Document Properties > Model Display > Reset To Defaults

It also behaves weirdly when Knitting.

DESELECTED

SELECTED FROM SURFACES FOLDER IN TREE

NORMAL SELECTION APPEARANCE

SELECTED WITH OTHER SURFACES

KNITTED

Many thanks!!

D

SolidworksUser Interface