>It might already be there? Just that the warning is so standard and ubiquitous that it is universally ignored (lots of things contain lithium and have the potential to explode).
No, the warning is not there.[1]
The manual instructs the user to take headphones off if you experience a "warming sensation", that's all.
The generic warnings aren't for normal operation; as in "improper use, like <...>, may lead to <...>". There was no improper use here, which is why it's a concern.
Additionally, there are many ways in which a device can fail. These Bose QC 35 II headphones not only caught fire, but also caused chemical burns.
Clearly, the engineers were aware of the dangers, but the product managers decided they can get away with saying "take them off if there's a warming sensation" without adding "BECAUSE THIS MEANS THEY ARE ABOUT TO CATCH FIRE AND SPRAY YOU WITH HAZARDOUS CHEMICALS, RUN FOR YOUR LFIE".
[1]https://assets.bose.com/content/dam/Bose_DAM/Web/consumer_el...
