Some domains redirecting to www. when there's nothing configured for that

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http://serverfault.com – I am having some troubles with a dedicated server, some of the domains in it are suddenly redirecting to the www. version of the domain, but there's no htaccess file or config in http.conf or /etc/nginx/vhosts/domain. The "redirect" is not even a 301 or 302, I am not sure what it is but the sniffer shows all the fields empty for the domain.com request and then goes right ahead to open www.domain.com where it received the server response. What else could I check to find what's causing this? (HowTos)