The Shadow Guard Headquarters is in Point Sur, California in an underground facility. There are several entrances: the helicopter area, vehicle bay and the boat launch.  A lighthouse sits over the area where the Shadow Guard Headquarters was built.

Click on image above to see Layout of facility.

If the Solider is married and has a family, it is most likely that they live in the Military community located in Big Sur, California. It looks like a private gated community with a large cement wall surrounding it. There is a sign in front of it, stating Huntington Villas. It is disguised as a normal looking Military family community.

No Dark One creature knows where the HQ is or has
 access to it by any means. 
If they attempt at locating it and happen to be near it, they will suddenly grow confused, disorientated, and wander around aimlessly. This is due to a spell protecting it from Dark Ones.
In order for a Dark One to be transported into the HQ, the spell will put that creature into a deep sleep until awakened by a counterspell.

(If you have a suggestion relating to this clause, please speak to a GM.)