HEADCANON:
Helena, like many other ghosts, is not just confined to the Hogwarts castle, as canonly proved by Myrtle being able to haunt Olive Hornby's brother's wedding.
I think it's kind of funny that the ministry technically has some sort of authority over Helena, (but i doubt even they know that she's Helena Ravenclaw.)
I imagine this was how she discovered that the diadem had gone missing, perhaps once visting it's hiding place, that or she could sense it the moment it arrived in Hogwarts via Riddle, though the former is more likely.







