Liz Buchanan BVSc MRCVS
Hello! I'm afraid that the short answer is that I don't know, but you can get some help.
Luckily, there has never been a better time to assess online specialists in a full spectrum of fields, including reptiles, because so many vets are now consulting online. The advantage of video consulting is that they can have a look at the pets' home conditions too, which are often extremely important when dealing with reptiles and have hitherto often been overlooked. Your local / registered vet will have some basic competency in examining exotics so it's worth calling them; if unable to help, they will know how and to whom to refer. In the UK, owners do have a legal and moral duty to seek out adequate veterinary case for their pet, but so do vets to refer appropriately.
Having said that, what could it be?
I am afraid that this picture is unclear - I am looking at some brownish marks on the dorsum of a blurred central coil and have no basis to rule out burns. Although I hear that you have a thermostadt, the sensors often monitor one point in the tank and do not rule out overheating in other areas of the tank (snakes may seek out the warmest). My understanding of heat guns are that they are not particularly reliable when assessing skin surface temperature, but please do speak to a snake specialist for that. I also wonder whether UV burns may be a possibility, as these do not depend on the temperature?
Good luck in accessing the help that you seek. At the same time as contacting a vet, top quality housing specialists can sometimes be very interesting to talk to. I would be interested to hear how you get on.