Batteries
Non rechargeable Lithium batteries will begin to self combust at well below the 500 degree temperatures reported as necessary to burn the airframe.
It could be hard to decide is a burnt ELT was the primary or a secondary source of combustion . . which may be why there is a bit of a delay
Primary Lithium Cells (Non rechargeable)
•Capable of self-ignition (thermal runaway)
•May worsen an independent controllable fire event •Violent release of a flammable electrolyte mixed with molten lithium metal (large pressure pulse)