It's quite possible that the CI_ENV variable is not set at all, in which case the code normally included in the public index.php file sets the ENVIRONMENT to 'development' by default. I set CI_ENV to 'production' in a conf file on my production server, but I don't set it in my development environment(s).
If someone else setup your site, it's also possible that they changed the way ENVIRONMENT is defined, though you should be able to find it by searching for "define('ENVIRONMENT'".
In the past, I've seen some pretty elaborate methods of setting this to avoid having to change the index.php file for each environment, since the CI_ENV check is relatively new.
