Security sign in notice not showing up in recent activityquestion

I awoke this morning to find a dialogue box telling me that I had been signed out on my laptop because I was signed in on another computer at 7:53 am. Since I was asleep at the time, I wanted to know how I might have gotten signed in, so I clicked on the "view details" link, which took me to the user center and the list of activity on my account. However, there was no activity listed for that time - the last one was four days ago (probably right). I scrolled through the activity - which goes back 5-6 weeks and didn't find any sign-ons for that time. Any ideas for the discrepancy? I used Air Droid on several different devices and may not have checked this laptop for a couple of days, so it's likely just me, but I'm curious why it's not showing up in my activity.