Mycroft Holmes: I have access to the top level of the MI5 archive.  

Mary Morstan: [already looking at that archive via her phone] Yep, that's where I'm looking.  

Mycroft Holmes: What do you think of MI5 security?  

Mary Morstan: I think it would be a good idea. 

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I don't know exactly what the last line of Mary Morstan means. 
Also, in that sentence, what does "it" refer to?