Typically in those days it was someone discarding fireplace ashes that weren't dead-cold and thus contained a smoldering ember. It's surprisingly easy to sweep out such an ember from what appears to be a long-dead fire, but you'll discover the heat if you sift through the ashes with your hand, even casually.
I swept some ashes from my (I thought) long dead fire from a few days earlier in there.
About midnight got a call from a neighbour ("your shed is on fire"). Fire department. Sirens. Two fire trucks. Major embarassment.
Kids somehow slept through the whole thing.
I lived in a ski resort town for some times, and the fire chief said that THE most common cause of fires was visitors conscientiously sweeping out the fireplace, putting the (seemingly) dead ashes in a bag, and leaving them out on the porch with trash collection — hours later, the building is on fire...
But honestly I've come around to a place where I think wood burning needs to be seen as a bygone era thing and probably quite regulated. I only do a fire a couple times a winter now. It's romantic and all, but it's polluting and of dubious safety.
What a bunch of BS "had to". US worships the automobile, so couldn't just dump it in the street, literally dump flaming garbage where kids play instead.
Some people are just lazy or don't know.
I do believe strongly in battery and electronic disposal. However, I'm not convinced that all disposal places are created equal. In my area, I will no longer use the hardware stores.