The help desk is reporting an increase in calls related to user accounts being locked...

A hunting hypothesis is a statement that describes a possible malicious activity that can be tested with data and analysis. A good hunting hypothesis should be specific, testable, and relevant to the problem or goal. In this case, the best hunting hypothesis from the following is that a password guessing attack is being executed against remote access mechanisms such as VPN, as it explains the possible cause and method of the user account lockouts in a specific and testable way. A zero-day vulnerability on a Microsoft Exchange server is too vague and does not explain how it relates to the lockouts. A hacked web application is also too vague and does not specify how it causes the lockouts. Users locking their accounts out because they recently changed their passwords is not a malicious activity and does not account for the increase in calls.