[To check the hallway, a mirror could work, too, and Mercury glances around the room for one, even as she thinks of other ideas. Failing the mirror, perhaps her ice could draw attention? She is, after all, no powerless puppet. It would be easy to know that had to be her doing, though.]

If the hall wraps around, we could use the angles if we could set mirrors at the right places. For now, we'd need to see right in front of the door.