Ah, yes, from the source code for the IM Transport service, we have something like:
So, yes, once you lose both Wifi AND Cell Data, you go offline.
MojLogInfo(IMServiceApp::s_log, _T("handleConnectionChanged no connection. setting all accounts offline"));
And it stays that way unless you do connect again.
It would be a nice addition for Sconix patches, I don't think it belongs to the messaging plugins