Semantic MediaWiki 3.0 started to use MediaWiki's extension registration 1 to load and enable its functionality. If for some reason SMW was installed but not enabled, Semantic MediaWiki 3.1 added some visibility to the Special:Version page 2 and shows a message to remind users about a potential missing enableSemantics configuration.
If the extension was installed but deliberately not enabled (e.g. for a specific wiki in a wiki farm setup) then it is possible to use the
$smwgIgnoreExtensionRegistrationCheck setting to suppress the check and notification on the Special:Version page.