I think they made some reference to it being due to the paint not bonding properly but to be honest I can't recall the detail.
What I do remember is them saying the warranty is not valid if the paintwork fails their original paint test or if the corrosion is found to be due to a stone chip or similar impact.
They also said they love this type of work and welcome any customer who brings in their car for assessment - obviously BMW pay top money to Sytner no questions asked.