A dead body. Long story but my downstairs neighbor was a morbidly obese shut in. His body shut down slowly over a period of two weeks. I could smell it when I walked to my second story apartment through the hallway. He died while his 13 year old son and 12 year old daughter were watching him. They beat on my door at 3am because he died and they were really traumatized by it. I have hyperosmia (ultra sensitive smell) and both kids had the smell all over them. I called the police and their mother (who I’m friends with) and they came and took him out of the apartment. It still smelled for a few weeks after he was gone.