Presence detection senses that a person or device is here, using a short range signal rather than a login or scan. It is the first step in recognizing who is nearest before anything acts on it.
The longer version
Presence detection asks a basic question: is a known device within range right now. A provider broadcasts a short range signal, and nearby phones that hear it report back. Detection on its own does not decide who is nearest or what should happen, it simply establishes that someone is present.
Detection becomes useful when it is resolved and confirmed. Many devices might hear the same broadcast, so a server compares the readings and settles on the single nearest party. Only a confirmed nearest match is reliable enough to act on, whether for access, service, or payment.
Keeping detection separate from action is what protects the person. In a presence based method a device can be detected and even matched, but nothing is sent until the person chooses to act. Detection informs, it does not charge.
The Parousya method
Presence detection is the sensing layer of the Parousya method. It notices that a device is here, then the server resolves the nearest pairing, and only a deliberate choice turns that into a transaction.