Update on the Great Tring Bird Robbery, courtesy of the Daily Mail. It seems that the theft was carried out by Edwin Rist, a 22 year-old American flautist and champion fly-tyer studying at the Royal Academy of Music, who conceived the whole escapade as a "James Bond" fantasy. In the words of his lawyer, "he did not use exotic tools to get in, in fact he smashed a window. He didn't even take a torch, and has described going around trying to get light off his phone. It was a very amateur burglary."
The lesson for us collections types lies in the reconnaisance that Rist was able to carry out - posing as an ornithology student from the University of Oxford, he gained access to the collection, checking out their holdings in the catalog, and under the guise of taking photographs of specimens was able to photograph the alleyways and corridors that he would use to get access during his break-in. Without seeming clever after the event, this is why you should always ask for references for students and do a quick background check before letting any visitors in (there's this thing called "Google" that you may have heard of).
Beyond that, I have a lot of sympathy with the staff at NHM. The reality is that most museums, especially the big ones, have nowhere near enough staff to constantly monitor the behavior of visitors while they're in the collections. I remember occasional discussions with the General Counsel at a previous employer on the subject of collection security; his opinion was that we allowed "far too many" people access to the collections. Obviously I disagreed with that, not least because while it's cheaper to try and deal with the problem of theft by choking off access, it does nothing for our fundamental mission of ensuring the utility of the collections. Ultimately you have to pony up for the staff or accept the occasional losses; locking the doors in the face of users isn't an option.