As a user of sogo webmail interface, I encounter a usage limitation in the global search fonctionnality. It appears to fully work for your own folders, but it stops on the "Shared" top folder and thus does not cover the mail folders shared with you. This Shared top folder is indeed virtual and it looks like sogo stop following the IMAP response that it does not exist. Maybe it would be possible to bypass this error to complete the global search over the underneath folders as seen in roundcube.