A bigger question might be if you found chametz over Pesach. Let’s say you sold all chametz in your home to a non-Jew. Then, you find something that you missed while cleaning – let’s say a cookie was found inside a cabinet. How can you burn it? It doesn’t belong to you, it belongs to the non-Jew that you sold it to!
The goy is not makpid (doesn’t mind) if I burn one of his cookies. If a person doesn’t mind if you eat his food (and you know this for certain), the halacha says you are allowed to eat it. The same would apply here.
But some say you cannot burn the cookie, but rather take a stick and push it into the goy’s section.