Can't find the Nissan procedure for refilling the 350z coolant (there does not appear to be a "How To" guide on here) but this guide supplied by Samco is quite helpful with the images - see the end part about the refilling steps:
http://crackaddict.c...samco.howto.htm
From previous posts over the years on here the refilling procedure, if not followed to the 'letter' has been the downfall of many and initially I was thinking of an airlock, although in your case the loss of fluid would suggest something else as others have suggested above, is amiss.
And you are best off using the Nissan Premix solution, should you have to drain and refill again - see here: http://www.350z-uk.c...genuine-nissan/