How much charge did your "new" MacBook have when you took it out the box?
Brand-new MBP's will be delivered with a battery that is approximately 50% charged (you can tell this by pressing the button on the outside of the MacBook on cMBP models, not sure if rMBP has one of those? or, of course, by plugging it in).
If the battery was fully charged when you took it out the box, it's a good bet its been opened - and run - before by 'someone'; whether that is an original purchaser, or a random QC check - who knows?
But in all honesty, does it make much difference? Were you prompted to set the computer up as a new one when you took it out the box?