Coolant leaking - head gasket?

I am losing about one liter of antifreeze every two weeks.I 've had this problem for about a year now. Nothing is showing on the ground. I found some antifreeze under the black air intake plenum where it connects to the engine. I looked at the water pump gasket, ( new one installed a month ago) themostat, and temp sending unit and can find no evidence of any leaking antifreeze. Anybody have any idea of where this antifreeze can be coming from? No evidence of antifreeze being blown under the hood, or any other engine part. Just under the black plenum

Thanks,

Jim