My girlfriend has an 05 Nissan Altima that I've been taking care of. I notice that about every 2-3 weeks the coolant resevoir is at the MIN mark or less. I add coolant to bring to max, but it goes down (about a pint) every 2-3 weeks. No leaks noted on garage floor... so, only other thought is it's burning it off. Head gasket? Runs great though, no smoke out exhaust. Any thoughts?