Prosocial behavior, the act of helping others, is essential to social life, yet spontaneous environmental triggers for such behavior remain underexplored. This study tested whether an unexpected event, such as the presence of a person dressed as Batman, could increase prosocial behavior by disrupting routine and enhancing attention to the present moment. We conducted a quasi-experimental field study on the Milan metro, observing 138 rides. In the control condition, a female experimenter, appearing pregnant, boarded the train with an observer. In the experimental condition, an additional experimenter dressed as Batman entered from another door. Passengers were significantly more likely to offer their seat when Batman was present (67.21% vs. 37.66%, OR = 3.393, p < 0.001). Notably, 44% of those who offered their seat in the experimental condition reported not seeing Batman. These findings suggest that unexpected events can promote prosociality, even without conscious awareness, with implications for encouraging kindness in public settings. Trial registration: ClinicalTrials.gov n° NCT06481748; registered on July 1, 2024.