Kotaku says that one user discovered Steam program files within the Punk Buster installation update for Battlefield 3.
A competition is also being held on the Sapphire Select Club website, entitling the winner to a Battlefield 3 download code redeemable from Steam.
Earlier this year, Electronic Arts confirmed that Battlefield 3 would not be released through the download service because its restricts the "ability to directly support players".
Speaking at this year's Gamescom, Valve head Gabe Newell said that he hopes to bring EA games back to Steam, but admitted that Valve couldn't "pick just one thing and think the issue would go away if we fixed that".
Battlefield 3 was released for the Xbox 360, PS3 and PC last week. The game shipped 10 million copies worldwide, making it EA's biggest ever video game launch.
Watch the 12-minute Fault Line trailer for Battlefield 3 below: