you have to assess how serious anonymous notes left in lavatories are. I would never give them much credence. Any serious incidents inevitably bring out the practical joke idiots for some period after, as may possibly have happened in London today.
If you are referring to the four bombs in London, they were certainly not practical jokes. Very serious loss of life was narrowly avoided. In light of which you may like to re-think your use of the word "never" in the paragraph above.