AFAIK: All the 406 ELT's use lithium batteries, not just Honeywell's. They are also in the ULB's (pinger) The FMS's for memory retention....they are all over the place. But the ELT is the only place I can think of where they are in a plastic box, not metal.
I obviously like the ELT theory, since it was my 1st thought, but I hope MurphyWasRight is on to something, that some other thing heated the ELT to the point of failure. The thought of an AD on every 406 ELT makes me shudder.
