Hi there, my Nissan Navara recently (3 months ago) blew a head gasket. There was a pipe that burst. I had it fixed under warranty and all was well. They skimmed the head and replace the radiator. But about a month ago whilst driving my Navara started to overheat. I decided to stop immediately and I called for Nissan's help. They ran all tests including pressure tests and what else and couldn't find a damn thing. However, the water tank and radiator was empty. They couldn't find a leak. After some anti freeze and water the Navara was good to go. And today I experienced the same thing. I picked it up through my aircon not heating up. Because the latter also happened when my Navara overheated the previous time. I opened my car and guess what? No water in the water tank and no water in the radiator. How is this possible? What is the problem? There is no leaking pipe whatsoever.

Please advise.
Thanks