The only thing that shows up, using the simplified search term "Office365" is the application itself, in the MS Office folder. No plist. Nothing in the global library, or the user library that refers to it.
The application name is Office365Service. It runs when one of the Office apps launches.
If the programs are launcing that service themselves, then I would think it would also affect my Mac, but it doesn't. So, I imagine it has installed a launchd item, but I don't have that item.
I opened Excel, Office365Service launched. Since I started writing this post, it has appeared and disappeared from Activity Monitor. It doesn't show up in the Excel Preferences, not even to connect with Office365 in the cloud. It may be a listener, that opens and closes, supposedly silently, to monitor those documents that you may want to push to the cloud service. Since it shows up only when it crashes, my guess is that the listener app is buggy (duh). There should be a way to disable it, or have it not load at all when the cloud service is not being used. A toggle in MS Office apps, maybe a global preference pane to disable it?