I have a Network Meter gadget running so I can watch the activity and there is nothing untoward going on. Some spikes of traffic, I am guessing just polling, but no constant background activity here.Many of today's DL'd games use some form of P2P and it's often not all that obvious how to shut it off. Don't remember which game it was, but when this first came out, disabling P2P severely nerfed available DL speeds. It's a way for distributers to save money on delivery of electronic media.
Best way to check for this is to "share" an internet connection with another PC. That way you can monitor bandwidth on the "host" to see if the "client" PC has any hidden activity going on. A higher end router should be able to tell you that as well.
I know Blizzard used a P2P style downloader for the updates of WoW... but I don't show any odd services or apps running either.