As
LivLion mentioned, it's a targeted offer; otherwise it would have been advertised openly. It's been around for a few weeks and yet another that bmi failed to hide from search bots. We had this conversation last month about other coded Silver offers, remember?
http://www.flyertalk.com/forum/bmi-d...-bmi-guys.html
http://www.flyertalk.com/forum/bmi-d...-one-year.html