Location:Middle of the Ocean That's why Buffy ask Willow to do one big teleport spell instead of opening a portal. Portals can be detected at both ends. Teleport leave only the first location. Twilight had to search the entire globe for them after the spell, so they had time to get rid of the magic, especially if the witches, the slayer-witches and Willow didn't do any more spells. If they had done more then one trip, Twilight would have had time to adjust his tracking methods, and follow the trail from the teleport point of origin and it's destination when they did it again. Their inherent magic was still detectable, and Twilight was shown searching for that. They were able to suppress their magic enough just in time. I'm looking forward to finding out how Amy was able to find them.