The "fault" light on the fuel mode selector push-button can appear to be dimly illuminated when it is in fact not. I can't remember the exact technical reason for this but it is a known peculiarity of the overhead panel. I have seen it reported in the technical logs on some of our aircraft by crews who didn't/don't know this - the response from the engineers is along the lines of that it is known and it is no concern.
Next time you see it, put the annunciator light switch to "test" - you will be able to see the difference between an actual "fault" annunciation and the dim illumination when it is actually off.