The quote suggests that individuals who are secretive or have hidden agendas often display behavior that makes them stand out, such as wearing sunglasses indoors. This implies that their actions may be a cover for ulterior motives, making them subjects of suspicion. The sunglasses serve as a barrier, preventing others from seeing their true emotions or intentions.

In the context of the book "The No. 1 Ladies' Detective Agency" by Alexander McCall Smith, this observation underlines the theme of vigilance required in detective work. It highlights how personal choices in appearance can reveal deeper psychological traits, emphasizing the importance of careful observation in understanding human behavior.