Once the coin reaches to the stomach most of the time it passes through the GI tract and comes in stool, unless there is some GI abnormality.
So watch the passage of coin by collecting the stool in container and running water over that.
Most of the time it comes in stool within 24 to 48 hrs. But sometime it may take 5-7 days. Sometime parents failed to notice the expelled coin.
Be watchful for pain abdomen, abdominal distension, vomiting
or any blood in stool
. Zinc containing coin may cause erosion of mucosa leading to some GI disturbances.
So I would suggest you to examine the stool properly and observe for danger signs if any.