Travel to the Manage Agents page.
Scroll right, to the Default Grid Visibility Setting column.
This column may be in a different order from here due to your ability to re-order columns.
Select the hamburger menu with the filter icon next to it.
The default filter is for hidden agents to not be visible.
Check the box for Hidden, and uncheck the box for Visible, then click Apply.
This will show only the Hidden agents.
From there, you may conduct normal agent management operations, including changing visibility.