Find my iPhone alert set off by someone else

The find my iPhone alert sounded on my iPhone 5S overnight. I didn't set it off, I immediately signed into iCloud on my web browser and only my devices were signed in and they were all accounted for. Two factor authentication was already enabled on my account and I changed my password within 15 minutes of the alert sounding. Any ideas how this could happen? Nobody else has access to my iCloud account, nobody was using my other devices (kids were asleep and devices put away, so it wasn't a kid playing with the Find My iPhone app), and I have never signed in on any computers outside of my own.

May be unrelated, but twice in the last month my Apple ID was disabled when I tried to sign in to iCloud.com and I needed to unlock it.