Actually you can place them any number of locations on installation. In addition Apple changed the default instal location I believe in on of the Final Cut Studio versions. There's also loops with GarageBand and it's possible you may have consolidated them as well.
One location would be
Macintosh HD>Library>Audio>Apple Loops>Apple>
and you may find several folders with loops. The current ones have a .caf extension. Some people will instal them on an external hard drive which you can hook up as needed.