DTOO131 - Trust Bar Notifications for unsigned application add-ins must be blocked.
If an application is configured to require all add-ins be signed by a trusted publisher, any unsigned add-ins the application loads will be disabled and the application will display the Trust Bar at the top of the active window. The Trust Bar contains a message informing users about the unsigned add-in. If the Trust Bar were to be allowed while add-ins are required to be signed by a trusted publisher, the user would have the ability to make the determination to allow an unsigned add-in, which increases the risk of allowing malicious code to run on the user's computer and on the network.
Set the policy value for User Configuration -> Administrative Templates -> Microsoft Access 2013 -> Application Settings -> Security -> Trust Center 'Disable Trust Bar Notification for unsigned application add-ins and block them' to 'Enabled'.