This is tough to trace, especially sharing. I believe inherited shares for example won't show in the sharing dialog box for the record, and this is likely where its happening, I'd look to parent record relationships, it could be caused by the parent being automatically shared with the original owner for example.
I think there are some security checking tools in XrmToolBox that may help you with this.
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You may want to try using the XrmToolBox with the plugin "Privileges Discovery". Using this plugin, simply select the "Read" privilege for the Opportunity entity, and it should show you all of the security roles that grant this privilege. Once you know which roles are granting the privilege, just check to make sure that neither the user or any of the teams that user belongs to has been granted a role that would allow him/her to read the entity.
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