'One cab ride huddled in the back attempting not to vomit later, Sherlock is standing at the doors for New Scotland Yard trying desperately not to pass out.'
One of the many excellent things about fanfiction is that it can provide wish fulfilment for both the author and the reader.
(Not like that! Minds in the gutter ...Although, there is a bunch of fanfiction about that sort of wish fulfilment too!)
So things that you won't get to see in the TV series or film or whatever - or things that are just hinted at - you can find explored in their full glory in fanfiction.