I believe for this model, which is similar to N Series model, you have to remove the keyboard in order to access the hard drive caddy

HOwever I may be mistaken but based on what you say, that there is no other panel on the underside of the laptop other than the memory cover, this would indicate the hard drive is under the keyboard

Most newer Sony laptops usually have a panel on the left underside which houses the hard drive and caddy

One screw holds the panel in place and three screws have be removed to extract the hard drive caddy

Hope this helps