I can guarantee you that the last time I tried to start Steam without any network connectivity it tried to connect, couldn't, and refused to start in that state. That was a couple of years ago, but it definitely used to be the case that the only way to get Steam to go into offline mode is to already be online. So now whenever I get ready to leave for vacation I make sure to take the laptop offline.
Likewise when Steam was offline this weekend (and it was only down for like a half hour), I would start Steam, it would go to "Connecting...", it would fail, it would bring up the login window with an empty password, and that was that. No way to login, no way to switch to offline mode. So it's possible that it saw the working network connection and decided that since it couldn't contact the Steam servers it wouldn't go to "offline" but I most certainly couldn't do it while it was out. (I think Steam was out in a weird way where the update servers were up and a few game servers were up, but the authentication and store servers were down.)
But I can guarantee you that there was no way to get into offline mode at that time. I suppose I could have tried unplugging my Internet connection but why would I have tried that when it's their servers that are down, not my Internet?