It may be under the ac compressor where you can't see anything. My upper rad hose was loose and coolant was weeping from it.

 

Release the serpentine belt and check for play on the fan no play the bearing is good, you cant see a leak but can smell it, there is a slight leak somewhere. hard to see once everything is on and around the thermostat housing, possibly small leakage at the end of the hose. or heater hoses going to the heater core dripping slightly on the exhaust, get a pressure check and pressurize the system you will find the leak then.