Pssst. It’s somewhere near this:

Horse Guards Avenue, London

(Picture by hugovk on Flickr.)

What’s this post all about? It’s part of a puzzle game in support of XDRTB.org, which is raising awareness of “extremely drug-resistant tuberculosis”. It was explained in a bit more detail on the Guardian’s PDA blog last week. If you’re in London, have a digital camera or camera-phone you can take part by finding 43 objects within one square mile…