Thanks Hikari, Carol and HerlockHolmes for your responses. I do understand your point that Sherlock underestimated the situation and young Openshaw didn't have much to worry since he was armed and went through crowded places. But the question simply arises to my mind because of the deductions Sherlock made.
Firstly he deduced that the secret lied in London so the danger was probably closer to him now than ever before.
Secondly he mentioned that the danger was not a man but men.
He also knew from Openshaw's story about the danger was real since these 'men' didn't leave any clue behind and were clear professionals.
Lastly he also deduced the 'deadly urgency' of the case since the letter this time was from East London.
But again as Hikari mentioned he's not infallible and has made some mistakes(or underestimated a situation) more than once. Also as I have got to know him more of a person who likes to verify his theories and do the ground work, this do seem a completely possible scenario.
Once again very much appreciate your views on this especially detailed one from Hikari.