A user that registered on our site a few days mysteriously disappeared 2 days later. Our site is synced with MailChimp through AutoChimp and we received an email from MailChimp telling us that a user had unsubscribed. Currently there is no public unsubscribe link to MailChimp nor have we sent out any email campaigns yet so the only way to unsubscribe is to delete a user from WordPress which automatically removes the user from MailChimp and triggers the unsubscribe email. Sure enough, the user was no longer listed in the users page. There are two admins, myself and my partner, and neither of us deleted this user. We emailed the user to see if she could shed some light on the situation but she didn't reply.
Any idea what could cause this or how I could track it down?
I have WordPress Antivirus installed running daily scans, and keep WordPress upgraded to the latest version. Just to be safe, since this happened I changed all our passwords (Admins and MySQL) as well as our authentication keys in wp-config.php. I also went ahead and installed Secure WordPress plugin, and set up WordPress Backup (S3).