Cole, who used magic to switch bags with Phoebe and Piper at a bookstore turns up at the manor and seems awfully keen to be invited in. He isn't, but he does get to shake Leo's hand from which he tell he is a White Lighter. Our suspicions as to Cole's true nature are further aroused when his shadow detaches itself and goes off about business of its own.
The Handfasting must take place during a solar eclipse to prevent detection from TPTB. Typically Piper puts helping others before her own happiness and the start of the ritual is delayed. Leo is twinkled away before the Handfasting is completed. In the last scene we see Cole reporting to a sinister council and it seems that he was the one who told TPTB about Leo and Piper's plans.