Private tasks - what roles can view them?

I have tried viewing private tasks (between other users) in both Administrator and also Full Access roles, but they still appear hidden.
What permission/role governs: can view all private tasks?

My concerns would be that users leave, with private tasks which need viewing by the boss (maybe the private tasks are the cause ), how can my boss see them (if not invited?) - after all, they are company information (ours Not Netsuite's!).
(apart from logging in as that user)

So far as I can see (I have tried some searches for private tasks under full access) I am unable in any role to get into private tasks (unless invited) this seems like a big headache to me, how can my boss review staff task lists - when they are setting tasks to private? - do I need to train users to invite the boss to everything they do? this will send me and them crazy.