Cast Detect Magic

You swirl your hands and shout "Revelara," and before you there lies a single magical effect: a strongly-glowing blue aura surrounding an area which contains a bunch of junk, a table, and a box which would seem just big enough to fit a key. But for now, you must deal with that aura. Hmm, blue means abjuration, which would likely mean the Alarm spell. You could either press on if you have what you need to deal with the box now or you could come back at a later time once you have what you need.