When you search for a document that exists but which you don't have access to, you should get some kind of message from the system confirming that it does exist, e.g.:
Your search for xxx matches documents you do not have access to.
Today you are just told that your search did not match any documents and the user doesn't know if it's because it doesn't exist or just due to lack of permission to see it.
This causes a lot of confusion for the users and a lot of unnecessary time spent to identify if documents exist or not.