For some time now, I'm noticing that a few of my Google Docs get accessed, yet I'm not accessing them myself, and they're not shared. I made a point of watching one of these documents and not touching it in any way, to see if it get accessed. Sure enough, it did get accessed.
I also noticed something similar in Gmail - a conversation that was marked as read, suddenly became marked as unread, even though I don't see any suspicious activity in the activity report that I get by clicking on the "Details" link at the bottom-right of the page.
Is this a known issue? Or should I be worried?