The mysterious smell at night

Hello, I'm writing to you with the hope of getting some kind of answer! Someone close to me, with whom I talk to nearly every day, often shares with me that when she's sleeping at night, all of a sudden she gets a whiff of a horrible reeking smell! She says that it doesn't happen every night and that there's been times when she hasn't felt it for months. It's also "passing" - she smells it once, i. e. it passes by and she smells it again in a few minutes! I don't know if it's a dead animal or something like that or if it's a mental health issue - what do you think?