In middle school one day there was a loudspeaker announcement that all teachers were to stay after school for "the special meeting, I repeat, the special meeting." It was so obvious to everyone that this was a coded message and we got it out of the Spanish teacher pretty quickly that this meant there had been a bomb threat.
They missed the only one I know in the UK - "Inspector Sands" I'm sure used to be associated with bomb / terror threats but wikipedia says it's just a general euphemism for something in a train/tube station that needs immediate attention from staff.
It was only after someone told me about "Inspector Sands" that I noticed how often it's used. Based on my anecdotal evidence, it totally makes sense to use all these codewords, otherwise half the stations in London would be closed on a daily basis :)