I haven't read the actual paper, just what I posted, so I can't say with any certainty. However, I believe that they are referring to the actual metabolic processes of the animal creating heat- the burning of glucose, etc. These reactions do produce heat; that is how our bodies do it! We just have regulatory mechanisms in place to maintain a set point:-) I can't imagine decomposition of prey, or a whole snake, maintain a temperature discernible above ambient, as the small amount of heat would be lost to the atmosphere very quickly without a layer of fur or leaves to act as insulation- the outside of the leaf pile isn't warm, only the inside!
Ryan- yes, locomotion would create small amounts of heat as well, in the same manner as muscle twitches for incubating pythons. I'm not aware of any snakes that produce light instead, but who knows what lurking in a cave somewhere with a phosphorescent glow??? I'd love to see that grant application:-)