motion detector

I set our alarm to AWAY when we left the house. After about 15 minutes into our drive, the motion detector went off. The alarm siren began. I got the notice about both on my phone which is all good but I don't know what set off the motion detector. We do not have pets; nothing fell from the walls, and the outer perimeter of the home was not breached. I disarmed the alarm and called a neighbor to check the house, etc. It was locked up tight.A few minutes after all this, I got a notice that the connection to the internet was lost, then about 3 minutes later, I got another alert saying it was now back online. Not sure if this had any impact on the motion detector since the internet connection was lost after the alarm went off and after I disarmed it. When my neighbor said everything looked fine, I alarmed the system again and had no further alerts for the next 4 days. What set off the motion detector? Any thoughts?