In one episode (ep 30) Toph was in a desert and she says she couldn't "see" normally, via vibrations on the floor. She said everything on the sand was "furry" for her. So basically she couldn't really "feel" Gaara's presence. Besides, Toph is a earth bender and a metal bender, not a sand bender. She tried to sand bend when the library was about to be buried in the sand of the de sert but failed, so in any circumstance, she cannot control Gaara's sand or feel it (or imperfectly becauseit's not earth, stone or metal).
If you don't belive me: watch the episode: http://***************/avatar-the-last-airbender-episode-30-english-dubbed
at 13 min 00 she said everything on the sand is fuzzy for her.
And at 18 min 28 she failed when she somehow tried her power on the sand.
Toph showed no issues in not only bending sand, but navigating through a sandy beach in book 3.