Had a portable bluetooth speaker do the same to me while I was in the same room with it, it was simply charging, wasn't even being used. Also very fortunate to be able to extinguish it right on the spot before it set the apartment and consequently the building on fire. I've grown pretty skeptical of that sort of batteries after the fact, but there's not much you can do about it at this point since there's one in almost every device you can think of. It's like sitting on an explosive device that can go off anytime without any notice.