Of course, it wouldn't be visible from outside the building. But anyone going there purely by curiosity will definitely know something's up there.
By the way, another argument about the temporary lodge:
Then spend a number of days equal to the lodge’s Force dedicating the space, setting up the physical components, building its astral form, setting up barriers, and harmonizing it to your aura. Once you’re done, your magical lodge is up and active.
Given the nature of the play, I suppose it could be reduced a bit, but you're still probably looking at a couple of days doing only that.