There have been 40,000 Warhammer 40,000 video games, but none I'm aware of revolve around the Adeptus Mechanicus.
The Adeptus Mechanicus are a group of humans who really, really love steel, technology and machines. They live on Mars building the Imperium's tech, while maintaining an army of their own. Basically, if Digital Foundry was a Warhammer 40,000 race, it would be The Adeptus Mechanicus.
So we come to Warhammer 40,000: Mechanicus, a new game published by Kasedo Games (Rise of Industry) and developed by little-known French studio Bulwark (Crowntakers). There's not much to go on right now, but we do know Mechanicus is a turn-based tactical game set for release on PC, Mac and Linux later this year. I'm hoping for an XCOM-style game, basically. Here are some pre-alpha screenshots: