The backup battery alarm is in fact a part of the Thatcham cat. 1 alarm. It is a siren with a little battery in it. It's placed behind the right front wing.
This alarm is only fitted on the ST models, and only in the UK. In the rest of Europe it is an option (only on the ST).
I retrofitted the thatcham alarm on my Titanium X, works great, but it is a lot of work to get it done.
Picture of the siren with backup battery: