The source is Gilovic, Medvecz,&; Savitsky 2000, Spotlight Effect in Social Judgment: Egocentric Prejudice on the Importance of One's Behavior and Appearance.

In the discussion part of this article (page 220), the phenomenon of bystander non-intervention mentioned in your post is mentioned. The reason is that when someone calls for help, we think someone should help this person. But everyone thinks so, so they are waiting for others to save lives. As a result, no one took action, which in turn gave us the illusion: it seems that the situation is not what I imagined, and it is likely to be a fuss. Otherwise, how could others not save the victims?

The essence of this phenomenon is that human beings tend to overestimate the perception of their behavior and appearance in the eyes of others, that is, egocentric bias, so they feel that all onlookers know what they are thinking. And when their expectations deviate, they will correct their original views according to the behavior of people around them.