apache2 mod_expires

I find that html pages are being cached without any directive for the mod_expires in the httpd.conf (We do not use .htaccess)
The only entry in httpd.conf is:

LoadModule expires_module modules/mod_expires.so

I am trying to figure out how apache2 is caching the html pages ie how is it able to cache the html pages and what defaults it is using.

Please let me know if any ideas.