Can't find backup files on Macintosh HD?
The backups are stored in a hidden volume called .MobileBackups. These are Time Machine's local snapshots. They are separate backups that save previous versions of files for cases where your Time Machine drive is not available. You shouldn't modify the contents of this volume directly, and therefore it is usually not visible in most cases.

You can disable local snapshots by running sudo tmutil disablelocal.

Re-enable with sudo tmutil enablelocal.