I would assume respiratory complexes I and III in mitochondria. Both used highly reduced states to create the gradient to pump protons, and electron leakage is inevitable.

This likely then leads to redox transitions in quinones, flavins, metal centres, leaving them in unstable excited states. When they relax, the excess energy has to go somewhere - usually thermal energy, but just occasionally, a photon.

This would also tally with anaesthetics and injury having an effect, as both effect mitochondrial function - and of course when you’re dead, so are your mitochondria.

Thanks, that's the first thought I had, thanks for reporting mitochondria. Which are not localized stuctures as pictured in undergrad courses.