Only you normally don't need to be online, as Steam has an offline mode?
Steam's offline mode does not work. It has never worked. It will never work.
The entire premise is fucking retarded - you have to be ONLINE in order to enable OFFLINE MODE. You have to KNOW that you're going to go offline in advance,
If you're not in offline mode and you have no internet connection and you launch Steam, it will simply fail to connect and you can't do anything with the Steam client.
If you are in offline mode and you do have an internet connection Steam will revert to online mode when it feels like it and start to do all the things you don't want it to do. And if a game has an update available, Steam will refuse to let you launch the game while in offline mode. Steam can learn of updates while in offline mode if you do have an internet connection, and lock you out of games.
If you are in offline mode it's just a matter of time before Steam decides you need to reauthenticate. Time, connection to a network (with or without internet connectivity), fucking connected bluetooth devices, etc. can all trip Steam's alarms and cause it to shut you out of your games until you reauthenticate.