If the fluid is low on the larger section of the master cylinder, it is probably an indication that the front disc brake pads are becoming worn. As the pads wear, the caliper piston moves out of the caliper, requiring more fluid to displace it. Since brake pressure warning and low fluid warning use the same indicator light on the dash, if the brakes are operating normally, its probably not a serious problem. If you had an actual brake system fluid leak, the brakes would not work properly(low pedal, mushy feel, etc.)
You should have the front pads checked, just in case they need to replaced now, rather than just refilling the brake fluid because if you fill it first, then find out the pads need replacement, when the caliper piston is pushed back into its bore, it will push the fluid back into the master cylinder, overflowing the reservoir.
Because brake fluid is hygroscopic, meaning that it attracts moisture from the atmosphere, the water and any associated corrosion/gunk it collects will settle by gravity to the lowest point, the calipers. So, you do not want to push that junk back toward the master cylinder(and thus through the anti-lock brake control), so the proper procedure to be used before pushing the piston in is to, using a brake hose pinch clamp, pinch off the hose, open the caliper bleeder valve, use a small diameter hose to direct the expelled fluid into a waste container. Then after pad replacement, pump the brake pedal to reseat the new pads against the rotors, then refill the master cylinder with appropriate fresh fluid.
Lastly, when there is a cold snap, the brake fluid will contract, many times causing the warning light to come on. Then after driving(due to heat under the hood warming the master cylinder), the light may go off, or you may see the light come on briefly when cornering. This is a normal indication of the aforementioned pad wear. Hope this helps.
on Feb 04, 2018