The concept of information loss is usually discussed with respect to a black hole. My understanding is that whatever matter you put into the black hole, it has only 3 "hairs" and so one doesn't know, just by determining the properties of the black hole, the mechanism by which the black hole was formed. There have been many developments with many people now believing that information is not really lost but gets mangled, etc.
Why is this loss of information not discussed in a far more pedestrian context? If you have a particle and an anti-particle annihilating into two photons, say; by observing the photons, you cannot reconstruct the velocities of the two particles. Have you lost information in this case? Is this concept of information identical to the Shannon definition? If annihilation is unitary, with entropy being conserved, I understand that Shannon information is also conserved. But, we cannot reverse-evolve to a unique initial state, can we? (Velocity isn't Lorentz-invariant, but, let us say that everything is carried out in a single inertial frame.)
More generally, I don't understand how information is not lost in so many processes that are many-to-one because of the nature of particle physics and why this is different from the scenario with black holes.