Yes your absolutely correct it should have serial number (as instituted by Colombo for all Cinelli bikes/frame). The story behind this frame is that three of these frames were found stored away after many years in the Cinelli factory in Milan. Someone I bought the frame from worked in the factory. Apparantly all three frames were not stamped with a serial number?. Enclosed is a pic of the bottom bracket.
I've been in the factory a number of times and actually emptied out a good portion of the "vintage" stock that was left there a few years back. Had the frame had a serial number, it would more than likely have been a warranty repair frame that had been returned with the customer getting a new replacement frame. According to what I have been told was Cinelli's old standard procedure, the returned frame was then to be repaired/repainted, after which it was to be placed back in stock. It is therefore more than likely that the frame was pilfered (happens everywhere, just ask a business owner) either in Cinelli or perhaps between a third party framebuilder and Cinelli.