Where are the project file backups stored in Logic Pro X?

Does anyone know where the project file backups are stored? There is a Project File Backups folder in the main project folder that appears to be empty, which gave me a scare until I saw my backups listed in the "Revert to" option in the File menu. The Sampler Instruments and Samples folders are also empty but the bounces and audio files are still where you'd expect them to be. So where are those project backups being sent? A Spotlight search didn't reveal the answer.