There is a single circular magnet about the size of a button battery. Its held in place by a plastic clip. As Pete says, if you shake the cassette too vigorously, it can be dislodged. If you're lucky, you hear it rattling in the bottom (unless you are accumstomed to the toilet contents rattling....)
If you do take it to bits, have a thin small size tie wrap handy-this will make sure it doesn't happen again-and you won't want that by the time you've done....