I don't know if the fire had been mentioned previously but it was definitely news for me when I watched the documentary. It's all very odd. I'm not saying I disbelieve the fireman - I've read in books that the smell of burning flesh is very distinctive and so I can believe that he would recognise it. I also find it plausible that he thought it strange, but that only when he heard about the case he decided to mention it.
But what I don't get, is what they could have been burning. Surely they wouldn't be burning her body 9-10 years after her disappearance??
So either they hid her body for 9 years, then decided to burn it
She actually lived a further 9 years and they killed her and burned her immediately
Or they were burning something/someone else
Either way, it's really bizarre