mysterious reboot and guest account active

This morning when I got, I found that my Mac had restarted and I was prompted for my login. However instead of seeing my single account, there was also the Guest account that was made active. Also when I then logged in again, my iPhone (which was connected to my iMac) prompted me asking I "Trust this computer".

I'm concerned that there may have been something malicious done to my iMac.

Any ideas as to what would cause this? Is this a cause for concern? Please advise.