When You Do Not Need Permission for a Background Check
As an employer, by law you must get permission from a prospective or current employee before conducting a background check. However, there is an exception to this rule according to privacyrights.org:

In California, if you have "suspicion of wrongdoing" you are able to do a check without permission. In other states, you can do a "misconduct investigation" without the employee's permission.

A misconduct investigation is conducted by a third party. You can order one if you suspect your employee of: 

• Misconduct on the job
• A violation of federal, state or local laws or regulations
• A violation of written policies of your organization
• Non-compliance with the rules of a self-regulatory organization.