Matza: The Search for Leaven
"On the evening before the first Seder (if the first seder falls on Saturday night, this is done on the preceding Thursday evening), after the evening service, the head of the household makes the final preparation for Passover by searching for leaven throughout the house.
It is customary to place a few pieces of bread in various places, so that when the search is made, leaven is found. Otherwise, the benediction recited before the ceremony would be in vain."
(Passover Haggadah. New Revised Edition by Rabbi Nathan Goldberg. Ktav Publishing House, Inc. Hoboken, N.J. 1993 by Asher Scharfstein)