When I hide an Item, can that Item still show up on Google?
Yes and no — it depends on whether the Item had already been indexed.
When you hide an Item, it is also automatically set to no-index (even if its Source Stream, or any of the Marketing Streams it appears in are indexable). This means that, from the point in time when the Item was hidden onward, it should not be indexed by Google.
However, if a hidden Item was indexed by Google before it was hidden, then that Item will still appear in search results until Google crawls your Hub again, which can take up to several months. If you need to remove the Item before this happens, Google provides a mechanism to do so: see this article from Google for details.