The fluid shouldn't drop even with worn pads. Check again and give us an idea of the thickness you still have. Also check if brake piston (?) is coming back completely after braking, could be griped. By the way do you have any braking problem?
Thickness looks to be about 3/8 inch. Not sure about the piston. All I can say is that the brakes are not dragging. The way I notice this was the brake lever traveled about 3/4 of the way in before getting any resistance. after a couple of pumps it came back up and stayed up. I had just come down a long twisty hill with 2 up, and ridden about 5 miles. I noticed no brake problems coming down the hill. The curious thing is where did the fluid go? Could it have boiled out on the caliper end?
OK ... I have the same problem with mine, once in a while it seems that there is a lack of fluid, then after pumping several times everything is back to normal. Seems it comes from poor brake line. When possible buy a braided line and it will be ok. When not possible (money), just get used.