Actually, Steam doesn't need to be online to activate offline mode.
It used to but Valve have long since held that it was a bug with the client. (Specifically, your local client tries to ensure that it and all of your games are the most up-to-date version on load. Previously if you had never played the game whilst connected to the internet - or had not done so recently enough - it was considered in need of a check with the server.)
But it was fixed donkey's years ago and personally I've had no trouble switching in and out of offline mode whilst not connected to the net ever since.
I distinctly remember, in fact, because it was my only significant gripe with the service.
I had seen a note in a steam update release note list a couple months ago (very roughly) that said something about offline mode had been fixed. So just tested it:
1) Grabbed my laptop, checked to make sure that steam was not open (it's currently logged in on my desktop). For what it's worth, it was running steam a few days ago.
2) Disabled laptop wireless adapter (no wire connection).
3) Opened steam.
4) Received this error message: "Unable to connect to the Steam network. 'Offline Mode' is unavailable because there is no Steam login information stored on this computer. You will not be able to use Steam until you can connect to the Steam network again."
5) Re-enabled laptop wireless adapter.
6) Opened steam again, got normal login prompt, logged in, worked fine.
7) Closed steam (and killed it in process explorer to make sure it didn't hang on).
8 ) Disabled laptop wireless adapter again.
9) Opened steam.
10) Received this error message: "Could not connect to the Steam network. It appears that you are not connected to the Internet, or that your Internet connection is not configured correctly for Steam. Either check your connection and click 'Retry', or start Steam in 'Offline Mode'. With a "Retry Connection" button and a "Start in Offline Mode" button.
11) Clicked "Start in Offline Mode".
12) Received this error message: "Could not connect to Steam network. This could be due to a problem with your Internet connection, or with the Steam network. Please visit www.steampowered.com
for more info." (kind of a funny poke to give a url to a person with internet connection problems, but that's by no means a gaff unique to Valve), and an "OK" button.
13) Clicked "OK" button and steam shut back down.
14) Re-enabled laptop wireless adapter.
15) Opened steam again, it opened fine without prompting for login since it still had the credentials.
16) Clicked Steam -> Go Offline -> Restart In Offline Mode
17) Hrm, steam crashed during the restart, but that doesn't normally happen so I try again and it works.
18) Close steam (kill in procexp from orbit, only way to be sure).
19) Disabled laptop wireless adapter again.
20) Opened steam.
21) Received this message: "Steam is currently set to be in Offline Mode. (blah blah)", so I click "Start In Offline Mode".
22) It works fine.
So I'm sure they fixed something, but they didn't fix whatever makes me have to start Offline mode while Online