Before the photon is detected it has no reality, because the photon, as an entity, is only equivalent to the complete emission-flight-absorption process we perceive. The true reality of a photon is the complete bridge it represents through spacetime. It experiences the whole process as a single event, so it can not exist partially.

When a photon appears on a detector, it is a one time event that is fully consistent with the evolution of the material world that allowed it to happen. A collection of all the spacetime conditions that made it pop into reality. An information exchange that happens through all the possible light-like paths that connect an absorbing matter particle to an emitter matter particle.