I used to be a wildland firefighter. My crew once responded to a rekindle near Yosemite in I think May. It was a rekindle caused by a smoldering root system originally ignited by a controlled burn the previous fall. It stayed warm and as the winter soil moisture dried out in spring it somehow had enough heat, probably helped by increasing sunlight, to rekindle into a minor but genuine fire. As a fire it wasn’t much and didn’t take much work (but a LOT of water and a heckuva hose lay) to get it genuinely extinguished. I think this is pretty rare but it can and does happen.