If there is no models in the scene, photon tracing pass will not stop

[TzuChieh]

Mostly because there is no surface for photons to land on. Photon tracer will keep shooting photons and cannot reach a certain number of recorded photons.

[TzuChieh]

Not easy to detect. Even if a scene has lights and objects, you can still easily setup the scene in a way that is impossible to find a landing point for a photon.