Your clutch works the same way as brake pads. As the brake pads/clutch plates wear, the fluid level drops to compensate for the lost material. I am really surprised that your clutch reservoir was practically empty with only 13K miles though. Did your Tech bleed the system, incase air was pulled into the line.Now this is the part I don't understand, the reservoir is completely empty but the clutch works great? So, I drive down to the dealership and we cannot find any leaks. Then, they top off the clutch fluid and we call it a day.