[flamberge]

But doesn't explain why presumed lithium battery in mystery drone didn't die.

Perhaps the "mystery drones" are using electric generators powered by small gasoline engines.

These craft are not hand-launched items you can purchase from Amazon. They are the size of automobiles, and larger. There are many alternatives for the propulsion systems.

[Windcatcher]

It could be a fuel cell, if the owners have enough funding.