The installation is hidden from the outside world in some way. It may be camouflaged behind a false facade, buried underground, or something similar. Note this is in addition to the Isolated feature, if any. An isolated headquarters is difficult to reach, while a concealed headquarters is difficult to find in the first place. Skill checks to locate the headquarters have their DC increased by +10. Each additional feature applied to this increases the DC +5, to a maximum of +30.
D20 Hero SRD Installations Article- http://www.d20herosrd.com/7-gadgets-gear/installations