If you find the fluid level low and with no leaks, you should go ahead and check the condition of the brake pads all the way around the car. As the pads wear, the piston moves out needing more fluid to keep the bore filled. If you refill, only fill to the min line until you get the brakes checked. When you replace the pads and push the pistons back in, the excess fluid will have to be removed from the reservoir.

James