The "old car syndrome" is that if its 110 degrees outside you will discover your Anti Freeze is not strong, your air conditioner fails and tires blow out in parking lots before you come home from the store. If its -20 below 0 F then the old car doesn't start, the water pump seizes or is frozen shut and the battery is dead. The common theme here is extremes, and how does a machine function under those extremes. It breaks at its weakest point and that is an old automobile at its weakest point, hot or cold. Vulnerable systems!
Now for the 787 Dreamliner and all its testing of its million parts, systems, and operational persons. What problems will emerge under the daily stresses of running an airline. You will find the weakest points under an extreme electrical system, dependent on a very robust electric architecture. These hundreds of thousands of parts are interdependent on successfully operating with each others, at points of functionality. When one acts up the other may overheat. Its not winter cold, or summer heat, but it finds the weakest link under extreme electrical use conditions, just as in an old car syndrome. It is the strong electric systems and new technology making the 787 an old airplane fast?
Honeywell has been called to London to advise and examine its transponder, located in the central burned-out area at the 787 top rear of the hull, where the fire's core burned and the transponder are both located. It is important to note this technology exist on all aircraft types, but are not necessarily run by a strong electrical systems such as Boeing has for its 787. That is an important clue in the LHR 787 fire. A robust electrical system on a Honeywell transponder is under examination. What happened if the transponder caught fire from electrical inputs; that may have shorted the device with its own Lithium Magnesium electrical storage system? Is this area encased like the main batteries? Does it have voltage regulation for spikes or anomaly current flows from the battery when the aircraft is at rest?
These are types of questions that will be looked into and answered with Honeywell shadowing the investigation. The kind of question levered out of the first events with its electrical issues, clear from the control panels to Lithium-Ion central batteries, and now further to the tail in an concentrated area with its subsystems; A Lithium-Magnesium battery supporting the transponder. Even though this battery type is already used in most aircrafts currently flying, are the safeguards in place protecting this smaller ancillary battery from 787's own electrical systems? Even if this is a wrong assumption, then its worth a check off point to consider anyways, then move on if it isn't the problem.
Otherwise, Colonel Mustard did it with the coffee pot in the kitchen.