For all intents and purposes the homeless were invisible. Or at least they were to the many Londoners' that rushed past them. They moved along as quickly as they could, never making eye contact or sparing a thought as to what would lead someone to choose to live life rough on the streets.
It was this aspect that made The Homeless Network the invaluable asset that they were. They saw and heard all manner of things that they weren't supposed to.
People were so careless around them. They didn't care what they said and did in front of them. The homeless they regarded as non-entities, having little to no value.
Sherlock Holmes knew better.
Being one of them for a time had taught him that.