Phantom o-ring after I put my car back together

I have a 2002 330Ci, manual. I recently did the CCV replacement (without removing the intake manifold) as well as the oil filter housing gasket, and removed the various cooling system components (radiator, expansion tank, etc) to change out the hoses. I put it back together, and noticed this o-ring (12mm, quarter for size comparison) underneath where the expansion tank goes. Does anyone have any idea where this belongs? It's not the coolant temp sensor or the
distribution piece of the intake manifold. Thanks!

Phantom o-ring after I put my car back together