Activated Carbon - Desorption in sunlight?
Your guess is right: the mechanism of charcoal regeneration is undoubtably temperature-induced desorption. Gas molecules are adsorbed on the large surface area of activated carbon by a mechanism of physisorption (or physical sorption) caused by weak nonspecific interactions (van der Waals forces) between gas molecules and the carbon surface. Unlike chemisorption, this process is reversible and, since it involves only weak interactions, it can actually be reversed by relatively small increases of temperature.

Wikipedia gives a specific example:


  The activated carbon can adsorb a large amount of methanol vapours in ambient temperature and desorb it at a higher temperature (around 100 degrees Celsius). During the daytime, the sunshine irradiates the collector, so the collector is heated up and the methanol is desorbed from the activated carbon. In desorption, the liquid methanol adsorbed in the charcoal heats up and vaporizes.


You will surely notice the resemblance between this text and your case.