Van overheating

Greetings to all fellas.
Since my last 500+ mile trip 3 weeks ago my van has overheated a couple of times. When I checked the water level (antefreeze) I have noticed that the radiator is almost empty, and I have not even done trips greater than 40 miles a day.
When I have checked, neither the oil rod seems to have water nor do I find a single leak under the car, except for this small drip on the exhaust side. What could be happening?
Thanks in advance ✌