I found my leak after I let the truck get nice and hot from driving it a while down the freeway then stopping at every exit and getting underneath looking for wetspots, or abnormally clean spots. You could have a small leak in one of the hoses that gets sprayed onto the engine block or the exaust maniflod and itll burn away before you can spot it. Also check the resivor lines for small leaks, as well as the heater core lines and the heater core.
You gonna have to go underneath the truck as well, rather than looking through the hood, to try to spot leakes.
Good luck!!