Ive been testing this for some time now (with a Necro only), Superior Sigil of Air (and i guess lesser ones as well) only works when placed in a single-hand weapon in the Main hand slot, like an Scepter, Axe, or a Dagger in the Main Hand, but it will not work on Horns, Focus, or even the Dagger in the off-hand.
More importanly, it does not work with Two-Handed weapons either, when placed in Staffs, Spears or Tridents, it never procs.