Did you look within your user-specific Library folder (aka ~/Library or /Users/<yourid>/Library)? It is by default hidden in Finder and (I think) excluded from searches. You can direct Finder there using the Go->Go To Folder top menu item.
If auto save is turned on, a never saved file will still auto save.
There is one unexpected thing though. When you reopen SketchUp and look in the recent files, the first thumbnails may be your most recently saved files, or an auto save of one of those previously saved files. The auto saved never saved file may be further down the list.
So, scroll down looking for a recovered file, named Untitled, and whose save time and date were quite recent.