It’s a strange issue because literally the only time SpamSieve sets the icon to hidden is if you uncheck Show menu bar icon in the settings. Yet there are a few of you for which it seems to be changing for other reasons. SpamSieve 3.0.1b1 adds some diagnostics to try to track this down (click to enable/disable). After you’ve updated and enabled the diagnostics, if the problem happens again, please send in a diagnostic report.