On my laptop, when it boots up, Steam always starts automatically. You have to have connected and logged on to Steam at least once to do this. So now, when Steam starts, sees no internet connection, but it tries to log in anyways, it will ask if I want to continue in offline mode. Click Continue or Cancel.
On my main desktop PC, Steam is always running, as is the PC. Only when I do a reboot, like after a Windows Update or program install does Steam ever shut down then restart.