I had recurrent low coolant and air pockets on my '13 V6. I never found an external leak. I checked the motor oil for water, I checked the exhaust for water, i checked the water for exhaust gas and never found anything. Then like a few others I replaced both caps even though they "looked good", filled it and purged the air one last time and I have not seen the problem occur again since then. Even the guy at the parts counter at the Nissan dealership asked me to confirm that I was actually buying two radiator caps -- they look the same but they are not.