It's a known fact that some DVDs have extra features. But on a number of discs you have to do some searching or
perform some kind of remote-control trick to find them.
In general, every "extra feature" (hidden or not) is advertised on the box. However, in some cases they arn't.
Now you could look for these trailers or bloopers. But if you don't know they're there, there is no reason to
and you might never see everything there is to see on your disc.
Can't find any? Search the web, there's a number of sites that have listings.
After all, a movie isn't an adventure game.