I think you have part of the answer in your question, which is that when a search box has the capability to return traditional search results or redirect you to a content page, then the user should be both notified and given the choice.
And in that scenario, one option is, as you say, to be able to give a notification along with the content page that this was a perfect match, but give them the option to see traditional search results instead.
The problem is that with this more powerful search funtionality, you are effectively giving the user one search result, making it seem as if actually there was only this one relevant result and that the search system is not as good as it really is.
I believe the level of confidence in the system actually drops when redirected to a content page: Is this really the only result? Is this the most relevant result. Was this all that could be found?
Another option might be not to redirect, but to separate the search results into two sections.
- Top section - 'Direct matches for your search terms' followed by links to the content page or pages - there can be more than one direct match.
- Lower section - The traditional search results
This allows the user to clearly see that there are both traditional results and one or more direct matches. The user then can see that the search system is smart and powerful - that it can differentiate the results by more than just 'containing the term'.
Similar in nature would be an initial list of disambiguation results (eg when you search Wikipedia for 'Orange County Airport') followed by traditional search results like you would find from Google
The further benefit in not redirecting is that the user always experiences a similar process (as opposed to sometimes getting a list of results and sometimes being redirected) even if there is indeed only one exact match. It may require the user to click on the link to go to that content page, but the user can see all the options - it leaves the user in control.