The product BS/EN/ISO etc. standard for ovens specifies the maximum allowable temperature for metal surfaces and control knobs. Samples of your Neff oven would have been type tested to the standard. I believe 'plastic' knobs are allowed to be 60 deg. above ambient, which I think - because of the thermal capacity of the material - is not regarded as dangerous.
Apparently many users are concerned oven panels or controls get "too hot", but it is the nature of an oven that bits get hot (but not dangerously hot for external parts!)
If you are concerned there is an electrical fault causing the heating, rather than it being due to the grill being on, then have it looked at, but I would expect the connections of a defective control to become hotter than the knob if the heat was being generated internally.