I don't know what's causing it, and I don't know where exactly it's coming from. But it's been lingering around the sitting room and kitchen for about a week, and it's starting to freak me out. Has Ju Ju puked or pooed somewhere invisible to the naked eye? Did someone leave a sandwich in a bag and leave that bag hidden somewhere around the house for weeks? The thing is, there really isn't anywhere for this smell-producer to be hidden - our house doesn't have that much crap in it, and I've looked under the couch and behind the piano and behind the bookshelves. And there's nothing! Except the smell! Which is a horrible sort of....organic smell. It smells like something going off, is what it smells like. And I can't find it.
I should note that this is not a repeat of the last time I was troubled with an unidentifiable domestic odour, in Berlin in 1995, which turned out to be because we'd left the washing machine stuffed with the wet clothes of the family who had sublet it to us and the clothes had all rotted. Our washing machine is fine. The smell, however, isn't.