For me, it's like stale smoke, ammonia
and strong ozone after a storm--all mixed tigether.
I had a very good air purifier on all night, right next to my bed. It did nothing. As I paint in oil, I use the purifier all the time. It always works. Not for this, though.
I'm at work right now, in a smoke-free environment. Still smell it.
I notice, too, that my coffee smells kind of rank when I have this phantom smell thing happening..