What do sociologists mean when they say that the homeless population of the United States is often "invisible"?

As Chapter 7 points out, public officials actively try to render the homeless invisible in certain situations by, for example, rousting them from public spaces before a major event is set to take place. Any answer should also mention that it is difficult to determine exactly how many people in the United States are homeless because the Census Bureau focuses on households when it counts the population. A good answer should also mention John Coleman's experiment as an "undercover" homeless man in New York City. Coleman noticed that most people tried to avoid even looking at him, and people he knew from his real life did not recognize him. This demonstrated that feelings of discomfort and guilt on the part of more privileged classes also contribute to the invisibility of the homeless.