How should I reply to 吃完了 (chīwánle = "finished eating") after I've finished eating? When I go to the cafeteria for meals, when I finish eating and return my tray the cleaners often say 吃完了 (chīwánle = "finished eating").  I'm unsure how to reply, and say 对，吃完了 (duì, chīwánle = "correct, finished eating") or some variant.

Question: How should I reply to 吃完了 (chīwánle = "finished eating") after I've finished eating?

I'm unsure of why they're pointing out the fact that I ate my meal (what else would I do with it?).
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.