Based on the Chrome extension's minified source code (via
https://chrome.google.com/webstore/detail/plus/bnebanooamokk...), with files like runReloadCurrentSnapshot.js, it seems that Plus's background worker is loading pages (possibly as pop-unders) in your browser as you use it, using whatever your current cookies & localStorage credentials happen to be.
What this means is that (whether now, or with an update that could easily slip the notice of Chrome Web Store auditors) Plus could direct your browser to take these actions on a domain where you had never explicitly told it to take a screenshot, using not only the credentials from when you installed Plus but whatever credentials exist on an ongoing basis.
Of course, this is also true of any extension that you grant permission to access all websites. But Plus has already shipped the code to access the DOM of arbitrary tabs already loaded in everyone's browser, and communicate that information to the cloud, without an auditable open-source core. I have a lot of trust, for instance, that if uBlock Origin were to start sending my data to the cloud, someone would post about it on HN. An attacker with the ability to send updates to the Chrome Web Store as Plus, and operate Plus servers as command-and-control servers, could do this a lot more subtly, and that's definitely a yellow flag.