Toyota Repair: coolant leak on 2004 v6 limited edition 4 runner, head gasket problem, coolant tank

Question
I brought my car into Toyota and they told me my coolant was just about empty and it looked like someone put some water in there.  My car is still under warranty and noone other than Toyota services my vehcile.  They said they could not tell where the leak was coming from, but did see some staining on the engine.  I brought my car back the next day for them to pressure test the coolant system, but they could not find the leak.  My "check engine" light has never come on as of yet.  Could it be a problem with a head gasket?  If so, is a dye test definitive in determining if it is a head gasket problem?  If it is definitive, shouldn't Toyota have performed the test?  There's obviously a leak somewhere if the coolant tank was just about empty.

Answer
Yes I agree, the coolant has to go somewhere, these engines are not prone to headgasket leaks or problems, how many miles are on it? A dye test is not usually the way to check for a headgasket internal leak, it should be checked by doing a chemical block test that checks for hydrocarbons in the cooling system.