I've got a 2014 Fusion SE with the 2.5 litre engine. I was replacing the valve cover gasket and while trying to pry off the valve cover, I broke off a small piece (maybe 1/2 inch square) of the plastic portion of the valve cover and I believe it dropped into the head. What is likely to happen if I don't find it?

 

. However, it all depends where it landed which is unknown. Myself, I wouldn't lose any sleep over it. I would continue looking for it as it probably is just laying there. If it landed in an oil passage, hopefully it will make it down into the pan where the object will just lay there harmlessly.
You might have to replace the valve cover now.

 

Yes, I have a new valve cover ordered. Thanks for your feedback!