Apple has not even tried to implement hiding. It is an intentionally missing feature.
What is called "Hide" in Photos is similar to the "Reject" in Aperture, removing a photo from the view, so only higher rated photos remained visible. There has not even been a Hide command in Aperture. And in iPhoto the Hide command did also not really hide the photos. Anybody could click "Show Hidden Photos" and reveal them or browse the Hidden Photos album.
The Help in iPhoto (http://help.apple.com/iphoto/index.html#pht6d60a948) says something similar to the Help in Photos. Hiding is meant to get photos out of the way, if we are not sure, if we want to delete them outright:
You can hide selected photos in an Event or album, or in a Faces or Places group, and show them only when you want to see them.
Hiding a photo can be useful if you don’t want to use it, but you’re not sure you want to delete it.
Nothing about keeping them secret and away from prying eyes.
In none of Apple's photo application is a method available to keep photos secret, not even in the professional application Aperture. This feature has been requested since Aperture and iPhoto came out, and it is still sorely missed in Photos.