Sounds to me like cracks in the seams/paint due to normal body flexing. Perhaps the normal flexible sealer has not been applied under the paint on these seams allowing moisture to get into the welds. Then again, the bodywork should be "galavanised". Not sure if the Clio shells are hot dip galvanised after welding or the panels are treated prior to manuafacture. If the later, heat stress during welding could still induce rust if moisture gets into the seams. If you have any pictures we can keep an eye on our cars for the same problem.