See Unlisted Videos Youtube Extension [patched] -

You can only find an unlisted video if someone else discovered it first and submitted it to the database. Alternative Ways to Find Unlisted YouTube Videos

While no extension can "crack" YouTube's servers to find every hidden video, they effectively bridge the gap between "unlisted" and "public" by indexing links found in the wild. For total privacy, users must utilize the Private setting see unlisted videos youtube extension