Your 2004 Silverado is using a lot of antifreeze but you are not seeing where it's leaking anywhere What could be the problem?

There is obviously a leak in the system. Take the truck for a spin, let it warm up and then leave it running in the driveway for a few minutes. Look for a drip. If you can't find anything, then your in more trouble. Shut the truck off, and check your oil for coolant because you likely need a head gasket.