A woman and her daughter go to Europe for vacation. They check in at the hotel, register, and then go into separate rooms to sleep. The mother sleeps late, and when she wakes she goes to her daughter’s room to wake her up. She knocks, and nobody answers. She turns the knob and to her surprise it is unlocked. She goes inside and finds nobody. She notices the room is very clean though. She goes downstairs, glances at the registry, and to her shock her daughter’s name is nowhere to be seen! When she asks the clerk about her daughter, the clerk says she (the mother) did not come in with any other person!

What happened to her daughter?