[p337]

If you use the drive integration, you share it with Slack. Slack then creates a thumbnail that is visible in that channel. Imagine pasting a sensitive HR document in the big company chat with everyone in it. No one in the group may have permission via Google, but they can see the thumbnail (and search its contents!) if they have access to the slack room.

Edit: I should note, this is my fuzzy recollection of how it worked 4 years ago when I reported it to Slack. YMMV