Passenger Information Units (PIUs) are specialised entities within aviation security that collect and analyse passenger data, primarily to identify potential threats such as serious crime and terrorist plots. They utilise the Passenger Name Record (PNR), a vital six-digit code on each ticket, to track and scrutinise passenger information. Operating quietly and invisibly, akin to clouds hovering in the sky, PIUs gather data from various sources, detecting patterns and anomalies that could indicate security risks. This article aims to shed light on their discreet yet vital surveillance, which plays a crucial role in safeguarding air travel, providing an essential but often unnoticed layer of security.