Happens to me once every few months. It always turns out to be a smell from outside that's made its way into my apartment. Usually someone's beat-up, overheating car being parked under my window.
Oh I know, I'm talking about the same thing. It usually happens to me when I'm sitting in my simracing rig which is next to my PC and a window. I'll be worried thinking my GPU or wheelbase has started burning when in reality it's an overheated car radiator or something right out my window.