I have seen this myself before, but its been when I've opened the same document by accident twice - I use different browsers and I had opened Chrome as well as Safari. Could it not just be something like that that has happened here?
Yes, I'm aware that. That's why I tried to think of scenarios where I was possibly using my other browsers/devices with the same gmail account. But, this is different. I just saw the same thing happening again with another gdocs document. The message comes up "hxxxxx has opened the document" and then some seconds later, "hxxxxx has left", while I wasn't doing anything. (If it wasn't due to some kind of anomoly in google docs, e.g., possibly as suggested spaghetti, then the only explanation seems somebody cracked my password, unfortunately.)
It's a long shot but perhaps an engineer was using one of the docs for testing? Or perhaps the notification was just sitting in some queue from hours or days ago and was finally fired off.
Could be. On your first note, I highly doubt that though. I worked at big companies who provided "cloud" services and I NEVER accessed customers' accounts. Your second explanation seems more plausible. I presume that's possible... Unlikely, but possible.
If you are serious about the security of your Google account, you should be using two-factor authentication. If you aren't, now would be a great time to start.
Thanks for your suggestion. It's like you think about getting a fire insurance after your house is burnt down. But, definitely, I'm starting to realize I have to pay more attention to security. This has been a surreal experience for me to say the least...