Not sure for anyone else, but here in Canada there is one central Nikon service depot for the whole country, and it is just outside Toronto. anyhow they do not advertise what they have in stock, but what I do is phone Nikon Canada (main number), ask to talk to service and repair, wait a second or three to get put through, and then ask what they have on hand for sale. Varies all the time, something of a crap shoot, and usually the guy has to put down the phone and goo look in back.
But any used/refurb item I have bought from them I have been very happy with. Prices are generally higher than used prices on the street or ebay, but then what you buy usually comes with a full warranty of a year, and you know everything has been cleaned, fixed, etc, etc. My D700 looked brand new, and seeing all the oil spot issues with the D600, I think I have had the last laugh. But it took a few phone calls in my case because the used and referbed D700s never seemed to stay in stock very long. wonder why.
When you buy retail "refurb" as other posters have pointed out, sometimes it is a return that a store cannot sell brand new but otherwise is. Not sure about cameras, but I have bought a couple "refurb" TVs locally with good luck.
In the specific case of a "refurb" D600, I would call Nikon direct (for whatever country you are in) and ask directly for Nikon service, and see if they have refurb D600s, because those cameras will - or should have been cleaned, inspected, etc, whereas a "refurb" from a large store might just be a repack of a cameras that was returned.
good luck