I know of one way you can get around it is to have yourself connected to something...just not the net. For example, if you are on wireless, you would be connecting to your router. That in turn would be (possibly) feeding your modem. If you don't have net, stay connected to the router and run steam, and it'll see you have a connection but no net, and will say 'fail to connect to steam, would you like to run in offline mode?'
Or, at least, thats what's always happened to me, I've noticed. Maybe it'll help you. You could also just set steam to run in offline mode right before you disconnect from the internet when you know you may not have access to it again for a while. Then when you start steam again, it'll automatically be in offline mode.