I don't think you could wear them down enough to use all the fluid capacity that you have in the rear reservior. I would suspect a leak or a bad master cylinder. Check the lines, there are a lot of them for the rear system. Check the calipers (all 3 since the rear is linked to the front) and check the bleeders, someone could have been an a$$ and loosened one off.
I wouldn't expect the pads to be worn out. But I suppose it's possible if you use one brake substantially more than the other that you might wear that set out first. If you're a rear brake kind of guy and use it as your primary stopping brake and rarely use the front brake, it's possible.