Sure they can 'sense' each other but only while they are in magical girl form or using their power. It is possible to hid from a Magical Girl you have never met, (Kyoko did it, I think) besides Amara has a plan, hehehe....

Edit: it is only confirmed that Contractors cannot be seen by normals.

Edit Edit: Besides, I felt it was an anime-ish thing to do. Maybe I have been watching One Piece too much...