By default, only Project Admins can see the Sandbox.

When you see three light grey boxes next to a red dot in the Permissions for the Sandbox, this signifies that these can't be selected. It doesn't mean that role has access to the Sandbox. A red dot means that content is not visible to the role. 



In the Sandbox, Reviewers with Manage Permissions can either have full access or no content access at all. 

A red dot means that content is not visible to the role.