When I make steak, I sear it on the grill at high heat, then transfer to a pan of already cooking onions, mushrooms, butter to finish on low. Sometimes I've left it a little too long in the pan and notice sear marks and some of the color has vanished. I notice something similar if I put sautee'd onions directly on top of steak; after a while, the grill marks disappear. Why?
My assumption is maybe there's an acidic compound in the liquid that leeches out of the onions when cooked, essentially washing away the sear marks. Or, perhaps with the butter for the same reasons.