Brakes

If you don't see a leak, check between the master cylinder and power booster to see if it's going into the booster. You might see black smoke from the exhaust too if it's burning in the engine.

I checked between the master cylinder and power booster..No leak there. Do the vehicle still have a wheel cylinder? The right front wheel looks a bit dirty compared to the other.

What led you to check the fluid level? Did a warning light turn on?

No. I have been noticing my brakes were spongy for about two to three weeks. But I kept driving. Just recently I was driving home one night and then the brake pedal traveled completely to the floor. I used the emergency brakes to get home. The next day I check the fluid level; completely empty.

That sounds like a relatively slow leak so any evidence of leakage might be washed away before you see any puddles. The rear wheel cylinders are a good suspect. Look underneath at the backing plates for signs of wetness. Steel lines can rust through too but typically that will show up suddenly. Check the front rubber hoses too.