But I fail to understand how smoke will enter if there is only damage and no fire.
Like wise. I fail to understand how any smoke, fuel, fire extinguisher, burning smells etc. could find their way against the differential pressure of a working bleed system v a system which is shut down and at ambient pressure.
It was explained to me that an engine with damage (i.e. no N1 / N2 or puncture in the casing giving a fire warning) will probably have metal debris from turbine / compressor blade rub / failure. If the metal particles get into the bleed system then the Non Return Valve function may have been damaged and cross bleed air could leak past the valve and into the damaged engine unnoticed (no bleed leak message). That could cause an excessive bleed demand on the working engine.