Willow had been gone for a year with no one knowing where she went

Without Buffy, Dawn or Xander knowing where she went, at any rate. Given that Giles apparently knows she's gone to Tibet and is astrally projecting there, I suspect that Willow talked to him about her "six-month mystical walkabout" but didn't discuss it with the rest of her friends. Giles may even have suggested the Tibet trip as a starting point for her to find people to teach her more about using her powers safely, and Willow went on from contact to contact until she eventually tracked down Saga Vasuki.