It’s true that the spam filter technologies in major email services have revolutionized in the past one decade. But since the spam technology is based on evolution, that’s it keeps improving its spam detection based on the experience it gains by detecting spams; hence there’s always a likelihood that it may tag a non-spam email message, as a spam.
This primarily happens because the email address from where that non-spam message is delivered, may have been listed as a source of spam by the Spam technology.
Hence make it a habit of scanning your spam Emails folder. And only when sure of their Spam credentials, delete them. It’s thus not advisable to Mindlessly empty one’s Spam Folder. A couple of minutes a day, can prevent you from deleting an important email tagged as spam.