You may have noticed that Steam's "offline mode" isn't foolproof, in the sense that it may periodically ask you to "check in" occasionally. Evidently, this is a bug, and not a feature, according to Valve on the Steam Community forums. Valve poster "hengryg" notes that "offline Mode is designed to be indefinite," and goes on to say that "the client should allow you to run in Offline Mode for as long as you like."
Not only should you report any issues you may have to Valve as a bug, but henryg also mentions that a new authentication procedure is on the way, making offline mode "more robust." A lot of us have been wondering what happens to your games should Valve go under some day. While that question may not be fully answered until the end times, it's great to know that there's at least something in the works to improve offline play.
How to have reliable offline mode on Steam [Steam Community via PC Gamer]