How is there an interference pattern already there, to serve as a guide? Possibilities: a). maybe from previous photon's travel using both slits but individually only one each. It requires interference of the effect of different photons and persistence of the disturbance; long enough to affect another. b) It is an effect due to the apparatus structure. Maybe miniscule vibration is enough to create 'environmental waves from the slits. c) An environmental disturbance (undetectable except by consequences) travelling ahead of the detectable photon passes through both slits/interferes, followed by detectable photon passing through just one.
Non of these have a photon taking two paths but could result in an 'as if ' appearance.