Selling chametz for a house which does not have chametz

Q: If someone does not have any chametz in his house, does he still need to sell his chametz?
A: Yes, because maybe he had has some and forgot. Maybe he has some medicine which contains chametz, some alcohol which is chametz, or a package of biscuits stuck in some corner. It is impossible to be certain, and even if he is certain, he should sell it.