I keep a boxed-in Snow Golem in my house. Twice, it has spontaneously vanished. The second time, I was around and I noticed that there was a pile of snowballs, as if it had been killed. However, I am reasonably confident in my house lighting and that there certainly wasn't a mob in the room then.
What exactly can cause snow golems to die?
The only discusssion of spontaneous despawning I could find on Minecraft Wiki clearly does not apply to Snow Golems since they last much longer than one minute.
(Please answer with experimental or code evidence (your own or a link), not just passing on what you've heard. There are a lot of urban legends.)