Studying shoulder surfing in natural settings presents substantial methodological challenges, including ethical considerations and preventing mutual influence between observers and those observed. This paper contributes to understanding opportunistic shoulder surfing in public environments, particularly public transportation, a vital space for diverse user groups. We first derive a research space on shoulder surfing to identify unresolved methodological issues. Then, we introduce our methodology for studying shoulder surfing in the wild using eye tracking. In our case study, participant observers wore mobile eye trackers during public transport journeys, allowing us to capture natural, often opportunistic, shoulder surfing behavior. From this, we derived valuable lessons learned and recommendations for future research. Our efforts deepen the understanding of shoulder surfing in real-world settings and pave the way for more effective mitigation strategies.