Unknown Bounce:

A unknown bounce indicates that the cause for the bounce could not be identified. It is normally caused by a technical failure, such as the subscriber's mail server being down as you are trying to send your message.

Because a unknown bounce is temporary and it is not a hard bounce, they are never removed from your active subscriber list. However, they will appear in Manage Bounces giving you a chance to batch or individually delete these addresses should you see they bounce more than once.