crawl under the truck and look at the bottom of the water pump. a lot of times the water pump seal will seep and the water is evaporated before it ever reaches anywhere. there will be a tell tale track of minerals from the evaporated water across the bottom of the pump snout. not usually visable without an inspection mirror or laying under it and getting the right angle to see. easiest way is to pull the water pump pulley ans get a good look.

also, you didnt mention if you have a sealed cooling system. from the factory they are not, and just burp onto the pavement. most of us have converted to a sealed system with the catch tank. if you are still on the factory open system, id check your rad cap. when they get weak, they let more water by as your driving and the pressures are up. youll never see it because your driving when its burping out. and of course with no catch tank, it cant get sucked back in later.