On Source Tree 188.8.131.52 for OSX, if my Repository Browser has no bookmarks, I have the option to scan a directory to locate repositories to add to Stash.
I cannot find a method to scan a directory if I already have bookmarks. The only way I've found to scan a directory is to delete all your bookmarks and then rescan, picking a directory that is a parent to all the repositories of interest.
This feels like a missing feature and the "Scan a directory" button should be available as part of the "+ New Repository" menu. If this feature is already present, it's location is unintuitive.