From the Stakeholder Search screen click on 'Show Filters' and select the check box 'Include Data From Other Projects'.
The results table will include Stakeholders from ALL projects.
Please note: records which do not belong to the project which you are logged into will only display contact details for the stakeholder.
To see further details including the interaction history you will need to select 'Share to this project'. Also, the delete function is not available if the record does not belong to the project which you are logged into.