The only reason the authorities knew there was a suspect device is because the Captain called them and told them so
Police said the pilot had been handed a note about a possible device on board - "suspect" not the same as "possible"
If there was any real prospect for trouble, why was this flight allowed to continue into Manchester, rather than diverting, as seems usual in such circumstances, into Stanstead?
There is a robust system in place for assessing the individual threat level.
PS correct spelling is "Stansted"