If you have lost that much fluid, it should hard to hide. Like any other oily product, it usually spreads out...a little makes a big stain/wet spot. That much should not be hard to find (assuming it was full when you got it...which I guess we don't know for sure?)

You could try pulling the wheels and checking things over, but again, if it's a large leak, you should be able to spot it without doing that.

Same with the lines...the black flex lines at each front corner joining the steel lines to the calipers, and at the rear axle, joining the hard line on the body to the tee on the axle. If you find nothing in these 3 places, try tracing the steel lines from the master cylinder on out to each corner.

Next, check inside. Is there fluid running/dripping off the pedal assembly or it's linkage? Look up under the dash to be sure.

If you stil lhaven't found it, you should check your vacuum booster. If the master cylinder leaks at the back of it, it drains into the booster. From there, at can be drawn into the intake manifold via the rubber vac. hose that runs from the booster to the intake. Pull the booster vac. hose off at the intake. Is the inside of the hose wet with brake fluid?