For a prompt: Please confirm that you are human below. Run with it.
Disclaimer: If I had any rights to the show, there would be a lot more gay sex.
At first, John doesn't remark upon it. It doesn't seem so very unusual, after all, to have Sherlock go straight to his netbook whenever they get back from a crime scene, not after they've just witnessed something odd and horrific and fascinating in the form of human remains. John assumes that Sherlock is making notes for posterity, or looking up random, arcane facts on the internet.
Then, when John hands Sherlock a cup of tea one night after a particularly gruesome murder, he notes the URL of the website Sherlock is visiting, though he tries to hide the screen.
John wonders whether there is a community for detectives on livejournal.
John pays more attention to Sherlock's internet habits after that night, noting the times when Sherlock will turn the netbook screen away from John and type for a few minutes, pausing every couple of seconds to jab a long finger at the touch pad. It never occurs after they have met with Mycroft, it always happens after they've been consulted by the Met, and occasionally it will happen after apparently random occurrences that John can't identify. There are times when Sherlock will appear agitated, others when he will appear calm, focused. There are times when he will stare at the screen blankly for several seconds before he begins to type, other times when he will type furiously right away, face set and determined.
When the opportunity arises, John has no qualms about the morality of stealing Sherlock's netbook. The curiosity has been driving him crazy, the dissatisfaction at having one of Sherlock's many secrets so blatantly displayed before him yet kept hidden distracting him at inopportune moments.
Sherlock is out, interrogating his homeless connections, and John approaches the netbook, lying closed on the table, and turns it on. It is surprisingly simple to access Sherlock's 'recent history'-John had assumed that Sherlock would either have deleted it straightaway or managed to hide it under layers of password encrypted documents—and John clicks on a URL for livejournal without hesitation.
The page loads, and John sucks in a breath. It isn't an incriminating blog post, no detective community. The page on the screen says 'Error. Your message was blank,' and then, underneath it, the polite request, 'Please confirm that you are a human below.'
Everything clicks into place—the frequent pauses (which must have been, John realizes, when Sherlock waited for the confirmation of his humanity before going back to the Error page and changing the words in Captcha), and the fact that they always happened after they met with the police (after they met with Anderson and Donovan.)
When John thinks about it, he is fairly certain that all the 'random incidents' that lead to Sherlock typing away furiously are instances when someone accused him of being incapable of soft emotions.
'Please confirm that you are a human below' John reads again, and smiles.
He won't ever mention that he has found this hidden part of Sherlock's humanity, he knows, (though Sherlock will probably deduce that he has from the fingerprints on the netbook) but it is nice to have proof of his flat-mate's humanity, knowledge to keep possessively in his mind and to hide behind a satisfied smile.
John exits the website before closing the netbook and placing it back on the table.
Where are you? John texts Sherlock, needing, quite suddenly, to be close to him.