2000 Ford Ranger hissing noise and loss of coolant

The other day after I was driving for about 30 mins, I parked and heard a hissing sound emanating from the passenger side front wheel well. I thought it was my tire at first, but then opened the hood and noticed that all my coolant was gone out of the plastic reserve tank. The sound seems to be coming from somewhere either near the top radiator hose but I can't feel any leaks anywhere on it. Also my truck hasn't overheated and there hasn't been any fluid leaking onto the ground.