How should I reply to 吃完了 (chīwánle = "finished eating") after I've finished eating?
You don't really have to give a formal reply.

Similar scenario: A Chinese arrives home and sits on his own sofa. His wife may see him and asks "回来了?". It's like "hi" in a specific context.

So next time you may just node and smile. Feeling awkward not to utter a word? "嗯" is enough.