Shulchan Arukh, Orach Chayim 446/שולחן ערוך·אורח חיים תמ״ו

1One who finds chametz during the festival [of Passover]. And it contains 4 sections: One who finds chametz in his house if it is on the intermediate days of the festival, he should take it out and destroy it immediately. And if it is on a festival day, he should cover it with a vessel until nightfall and then destroy it, because he cannot move it on a festival day (Ribash). Also, to burn it in its place is forbidden (Ran).

2And there are those who say that the second day of yom tov has the same law as chol hamo'ed regarding this matter.

3If the roof of a non-Jew was adjacent to the roof of a Jew, and chametz rolled from the non-Jew's roof onto the Jew's roof, one may push it away with a stick. But if it is Shabbat or Yom Tov, one covers it with a vessel (because it is forbidden to move it on Yom Tov and Shabbat).

4If one finds bread in his home during Passover and does not know whether it is chametz (leavened) or matzah, it is permitted even to eat it, since we follow the most recent presumption — unless it is so moldy that it could not have become that way since Passover began, in which case it is certainly chametz. However, if enough days of Passover have passed to account for the mold, and it is one's custom to bake fresh warm bread every day during Passover, we lean toward leniency even if it is very moldy, attributing the mold to the daily placement of warm bread on top of it.