There is a setting that via Tech2 changes the fog light position from left to right.
You only need one and it has to be the off side one.
Its the same for the continent so rather than make a left hand and right hand light clusters they just change the settings
If the brake fails its actually the side light bulb that takes over with a higher voltage to make it look like the brake light, not the fog light that illuminates
If you activate the abs you get six bulbs, all 21w all flashing. Having had four facelift Vectra Cs, I can't see the Signum being any different. Put another way, if it never illuminated, it would never have a bulb fitted 
Please read the manual - fog light position is country specific, opposing side can be used as a spare bulb holder - fog light does not illuminate in event of brake or side light failure, cheaper to have some donkey in a factory installing all bulbs at a few pennies each rather than having piles of country specific units cluttering up half of Germany.
Having owned 2 Signums, a pre facelift and a facelift, neither had a bulb in the near side fog light, both have the tail or brake light work as dual use in event of bulb failure