Think about that a minute. Mycrft either sneaks into their flat himself, reads their scrapbook and instead of seizing it or burning it himself in their fireplace... he writes out a little sticky note and leaves it for them to find. That, or he has a minion break in and read the material, steal the scrapbook which Mycroft looks over in the comfort of his office, does exactly that with the sticky-notes, and has said minion sneak it back into 221B. Maybe he instructs Anthea what exactly to write out (though the handwriting looks masculine). Maybe Anthea knows him well enough to carry on his side of the Sherlock-Mycroft bickering while he's off negotiating trade agreements in Budapest. Maybe Mycroft prepares the sticky-notes in advance and sends them with said minion because he knows what John will put in the scrapbook.
Really, I'm struggling to come up with a scenario that doesn't have me giggling like a wildly giggling thing. It's hilarious.
And John keeps on doing it. Canonically, as late as HLV, John is simply not intimidated by Mycroft. I love imaginng him as a smooth, suave kingmaker but it seems equally funny to imagine him as a horribly inept public servant. Or at least one who is the true drama queen and who takes a little too much fun playing around in his brother's private lives. At best, it's a really very odd hobby.
Now I'm imagining Mycroft fiddling with traffic lights just for the hell of it, because he enjoys seeing people frustrating. Also, Anthea standing behind him and viscerally trying not to roll her eyes. Because he's just that kind of a man.