A more plausible source for a leftover legitimate copy is the second-hand market. One user in a Steam forum claimed to have bought an "Amazon version" of the game from the key reseller Kinguin for $20. This version, they say, "comes with online activation code, uncut". However, there are significant risks with this path. The validity of the key is never guaranteed, the activation servers may be offline, and it's impossible to know if the seller is offering a unique unused key, a pre-activated account, or simply a pirated copy packaged as a "key".
Standard serial keys printed in old manuals or generated offline will fail the modern online authentication checks required by the original, unmodified retail installer. How to Play Manhunt 2 on PC Safely Manhunt 2 Product Activation Code Pc