Should Cinemas Let The Audience Know If A Film Has Scenes After The End Credits?

[powerranger69]

I watched Watchmen on a boring Tuesday afternoon in my local Apollo cinema, and as soon as the credits started rolling, not only did the lights go to "cleaner" level as you say, the actual bloody cleaners came in and started working
Ok there were only about 7 people in there, but I for one wanted to watch some of the credits and they frakkin ruined it