Yes i know about those impurity things, but that's not my point. I was questioning about how did she manage to locate the metals on the first place, since she only relies on her feet to feels the vibrations on the ground to locate things. By this way, a drop of mercury that came out from Korra's arm, floating mid-air, should be undetectable to her (that's why i wrote "levitating" with caps on).
I hope i explain it well to you, this is not my native language.
Toph's search to find spiritual enlightenment has super amplified her seismic sense to the point where, not only can she sense people all over the world from The Swamp, but can also sense trace amounts of solids levitating. She is able to sense the exact position of the mercury by sensing exactly the kind of metalbending motion Korra is making and the direction and power Korra is putting into the movement.
No, she could always sense earth in the air, she does it during her introductory fight. Also, I'm not sure if you're aware, but she can only "see" across the world from the swamp, once she leaves she doesn't know what's going on, as evidenced by her not knowing where Suyin was taken. It's a quality of the swamp's spiritual powers.
Remember the episode The Desert? She kicked a boat hard enough, she caused enough vibrations to know it was a boat under the sand, even though the sand was just a giant wad of tiny stones. Sight is just in her feet and Earthbending is an extension of her senses.