For clarification, the fire detection loops do activate both the overheat and fire warnings, just with differing temperature thresholds. None of the detectors are inside the engines - they are around them. They detect heat build up inside the nacelle, not inside the exhaust or turbine, and so have no relationship with engine DU indications. An overheat warning can be triggered by a bleed air leak, as suggested, but also a core leak (combustion gasses leaking between combustion, turbine and exhaust stages) or a small fire. It is a very serious condition.
I take the comment about further effects including structural integrity of the pylon, too. While one might anticipate a bleed duct warning instead, that may or may not happen - a turbine leak could end up going all sorts of places, especially once things start burning or melting away. Even if the pylon was unaffected, the structural integrity of the nacelle could be affected.