How can i display a environment indicator notice on my dev and staging when browsing the site?
You can't output HTML (or text) directly from settings.php without throwing an error, since settings.php is interpreted during bootstrap.

Provided you have a clean install, on your staging site, have the following line in settings.php:

$conf['site_name'] = 'This is a staging site';


All assignments to the $conf array you do in settings.php will override whatever value you set the in the administrative GUI for the site.

An alternative solution, if you don't have a clean install and the site's name is no longer displayed by default, is to add your own conf variable to settings.php:

$conf['is_staging_p'] = TRUE;


and modify your theme to output some indicator if this predicate is TRUE;