Private Browsing mode should prevent most Web browsers from saving loaded content in any way. Any such information such as that in your browser’s cache, its history, or cookie information should be stored temporarily for the current session, and then discarded when you close your browser window. However, in Safari your pages might be logged by a small but overlooked aspect of how Safari handles Web pages.
As described by MacIssues reader Tyler C.:
“I was troubleshooting some issues with Safari and opened up ~/Library/Safari/WebpageIcons.db, and in that file, I was surprised to find all the URLs of sites (stored in plain text) that I and others using my Mac visited in “private windows” in Safari. The URLs seem to stay in there basically forever unless you clear out all your browsing data (which defeats the entire purpose of using a “private window”).”