Evidently, Activision has hired a firm called IP Cybercrime who deal specifically with tackling online piracy. Instead of going in hard, however, and perhaps trying to sue the pirates, the company is taking a different tactic - approaching sellers of the disc, asking them politely to stop and trying to find out how they got their hands on the leaked copy.
IP Cybercrime has reportedly achieved some success so far using this tactic with a number of pirates immediately giving up their copies, including one YouTube poster going by the name of "Computer Healer" who said he bought a copy for $150 and surrendered his copy after being approached by the company.*
IP Cybercrime uses a variety of ways to track pirates, including monitoring Facebook and other social applications and can also apparently track down leaked copies via Xbox Live.
* The YouTube video has now been removed.