As said the waking sands has had magic cast on it since the attack, anyone who enters into the building who isn't supposed to be there (unless they can see through the deception i would guess) will just see an abandoned building, even if a scion was fallowed in the person fallowing them would likely find no one inside.

I don't remember exactly who explains that but it is in-game. But even then in terms of story the scions don't stay there longer than it takes to get a new HQ and start working on getting rid of the garleans. Not to mention that the base holding the scions got something of a reckoning when you broke your fellows out of it, you can pretty much stamp "caution is advised" on future dealings between the scions and garleans.