I lived in an apartment building years ago where the cops had to break in a door to discover a dead man. Everyone that walked into that building KNEW it smelled like dead human. It wasn't merely heady whiffs of the cadaverine/putricine/etc. I think there's something inborn by which we are aware that a certain smell means a human kicked the bucket. I'd never smelled it before that day, all the dead people I'd encountered were in funeral homes, etc. But as soon as the smell hit my nose I knew what it was even though I'd never smelled it before.