I’ve been procrastinating a bit lately and re-watching ‘Sherlock’ during breaks in rehearsal at work. I’ve haven’t really been keeping up to date with the ‘Sherlock’ community so not really sure what the current popular theories about the IOUs in ‘The Reichenbach Fall’, so here is my theory…
They aren’t aimed at Sherlock, they’re aimed at Mycroft.
Fandom has pretty much decided Mycroft is watching Sherlock constantly through the CCTV network but I think he’s got better things to do with his time than follow his little brother around London. I would put money on Moriarty being under maximum surveillance following his acquittal, and all that ‘GET SHERLOCK’ graffiti in his cell would have unnerved Mycroft.
If you think about it, he is one person that Moriarty really owes.
The graffiti is to antagonise Mycroft and taunt Sherlock. Sherlock notices it properly in the office building opposite Scotland Yard and I think this is when he starts to piece together what’s happened.
However you look at Mycroft, one thing seems pretty clear – he’s isolated but still part of Sherlock’s circle (however separate) and I would bet that pre-Reich if Mycroft needed help, Sherlock, John and Lestrade would be there (willingly or not). Post-Reich, I doubt they’d be as interested in helping him as he’s (inadvertently) cost Sherlock his ‘life’, risks John’s mental health and Lestrade’s job is probably at risk.
Perfect time for Moriarty’s gang to attempt to bring down the British Government. However, I doubt they’ll get far…