Rather than discuss something I don't know or understand like many of the poster's above (who seem to be leveraging former experiences with prior OSes) I will state my experience and you can accept it for it is.
I purchased an ACER M5 with a pre-loaded OEM version of windows 8, similarly there was no affixed sticker stating the product key. I removed the original hard drive and upgraded to a new SSD. No efforts towards imaging or maintaining of the original HHD were performed. I purchased a system builders DVD for windows 8 pro to install onto the new, entirely blank SSD. During the installation, I was never asked to input a product key. My final istallation was a copy of windows 8 (non pro). Futhermore, a check on the last 5 digits of the product key applied on my installation reflected a different product key than the one shipped with the system builder DVD.
Speculation: the OEM product key was stored elsewhere on the machine, detected, and employed rather than asking the user to supply a product key. I've read elsewhere that the key is maintained in the BIOS for new OEMs from major PC manufactures to simplify reporting to Microsoft.
Personal: I build a new box every two years so the fact that my product key was never requested doesn't impact me; my purchased personal use license for W8 pro will be used eventually.