1995 Ford F-150 Radiator Fluid Status

I noticed recently when I filled my windshield washer resevoir that my coolant resevoir was empty. I filled it up to the cold level mark on the side of the resevoir with coolant and the truck engine cold. I have checked it several times over a couple of weeks and the resevoir is always either empty or almost empty. So I refill it again. I haven't noticed any leaking on the ground or inside around the engine or antifreeze smell. A new radiator and hoses were installed about 5 months ago. I haven't had any overheating warnings from the lights or the gauge either. What could be the problem please?