I've started to use the OneNote Web App, and I think Microsoft have made a very good job developing it. I use it on the computer and on the phone with ease. I use the keyboard shortcut ctrl + S to save and expect the document to be saved. After using it a while, I started to browse the ribbon navigation bar and found that the save-button is omitted.
Pressing the question "Where's the Save Button?" gets you to a popup that does explain why its gone:
That’s interesting. Especially since we on the User Experience Q'n'A at Stackexchenge often answer questions with conventions. As in the famous save icon question  where the accepted answer referred to an idiom.
However, I want to know which usability research Microsoft based there decision on to omit the save button. I've searched but didn't find anything. Not even on Microsoft official OneNote blog , where one would expect to find background information. This leads me to asking:
Based on what usability research did Microsoft omit the save button on OneNote Web App?