I have a sphere or a box with ideally reflecting sufraces (inside). I open a cover, catch 1 photon, and close a cover. Photon is infinitely reflected back and forth inside. I can open a cover later and analyze it. Now say I can a 'superposition' with 50% of the chance that I have a photon and 50% that I dont have it. Even photon is bounced inside in reversible manner, I see 2 problems: 1. Photon inside increase the energy stress tensor, so very sensitive device can detect extra gravitational attraction from the box, revealing 'which path' information. 2. When photon is inside the box, the box is under additional stress. As there are no absolutely rigid bodies, box becomes slightly larger. Also, it information is not elastic, then the box becomes hotter, so it is detectable from the outside. So, even mirrors are ideal, still ofter some time we will be able to find out if photon is inside. What is a treshold?