If you've lost sleep over the absence of Aliens: Colonial Marines and the 2010 version of Alien vs Predator on Steam, then it's time to relax: they're back. Both Sega-published games disappeared from Steam and several other services late last year, and while no explanation was provided it was likely due to some dizzyingly complicated licensing conditions.
You're probably not overly concerned by the disappearance of one of the most notoriously poor video games of the last decade, but sometimes when licenses expire, games disappear from digital storefronts for good: a deal between Activision and Marvel expired at the start of 2014, leading to the removal of, um, Deadpool. It's still not back.
Here's hoping this doesn't happen, even temporarily, to Alien: Isolation, which took our Game of the Year gong for 2014.