Steps to reproduce the issue

I've tested new version of website had to make website offline and work on its SEO and to validate URLs in content. I turned website offline in global configuration, then log in a front-end under super administrator's credentials and tried to enter any wrong URL.
Correct url: /about-us.html
Incorrect url that I'm trying to enter: /about-us222abcd.html

Expected result

I must get 404 response code when input wrong URL.

Actual result

Redirect at index.php with 303 response code.

System information (as much as possible)

I tested it with Joomla 3.4.5

Additional comments

It's happen when website is offline and because of these .htaccess rules:

Begin - Joomla! core SEF Section.

RewriteRule .* - [E=HTTP_AUTHORIZATION:%{HTTP:Authorization}]

If the requested path and file is not /index.php and the request

has not already been internally rewritten to the index.php script

RewriteCond %{REQUEST_URI} !^/index.php

and the requested path and file doesn't directly match a physical file

RewriteCond %{REQUEST_FILENAME} !-f

and the requested path and file doesn't directly match a physical folder

RewriteCond %{REQUEST_FILENAME} !-d

internally rewrite the request to the index.php script

RewriteRule .* index.php [L]

End - Joomla! core SEF Section.

Please take a look this issue happens when website is offline and you're logged in as user with enough permissions (administrator and above) at front-end side. URLs should work correct in offline too.