Does anyone know if there's a similar risk with rice bags, and if you're OK as long as you only set the microwave for a short period? My rice bag is quite small, it's a sock in fact, and I don't heat it further than warm as it's used as a warm compress over my eyes.

I once made a nice wheat bag for my mother, and it got accidentally killed by mistake. She has a combi oven-microwave and left the wheat bag in there one day. I didn't realise it was in there and preheated the oven to the highest heat for a baked potato. By the time I found the wheat bag in there, it was thoroughly charred. No fires, thankfully.