Question

In the laws of the Sabbath there is a rule that it is forbidden to insulate food in something that adds heat even on Sabbath eve.
I am puzzled by the concept of "adding heat."
Is there any reality in which something becomes hotter without an energy source?
I can think of radioactive material as something that adds heat, but olive pomace?
Or perhaps olive pomace develops some kind of fermentation process that can heat the dish?
I thought maybe the Sages described, in scientifically imprecise terms, a material in which, if one insulates food in it, the heat is preserved optimally as if it were on a fire, and they called that "something that adds heat."
But from the wording of the Shulchan Arukh (Orach Chayim 257:1), which says: "And if one insulated it in something that adds heat, the dish is forbidden even after the fact. And this is specifically when it was cold and became heated, or when it shrank and this improved it; but if it remained at its original warmth, it is permitted"—we see that it does not merely preserve the heat but actually heats it up.
I would be glad for his answer both as a man of Torah and as a man of science.