A while ago I came across that some confidential material appeared on my search results. The material was located on a site which was already deleted a long ago (yet still the search result showed its preview, although once opened the "not found" error appeared). The site had unique user permissions, but the search results were now visible for all users - also for the users that never had an access to the mentioned, already deleted site.
Although both incremental and full crawls had already ran multiple times since the deletion of the site, I ran them in attempt to hide the results, but it didn't work. I also tried the commands mentioned here and then ran the crawls.
Finally I did an Index Reset as suggested here. That worked and the results were gone for good.
Although there's a workaround for this issue, it isn't enough. To avoid this issue happening again I need to find a way to either automatize the Index Resetting (+ de- & re-activating the Search alerts) or an alternative solution for how to make sure this doesn't happen again.