Pesachim, Chapter 1, Mishnah 2. The Mishnah turns to a practical detail in the search for chametz, teaching that once a person has completed his search he need not worry any further. What was there to worry about?
The words of the Mishnah:
"Ein choshshin shema gerarah chuldah mibayis l'vayis u'mimakom l'makom" - we do not suspect that a weasel or a mouse may have dragged chametz from a house that has not yet been searched into a house that was already searched, or from a place that was not searched into a place that was.
The reason - "there would be no end to the matter":
The Mishnah explains: "D'im kein, mechatzer l'chatzer u'me'ir l'ir, ein l'davar sof" - if we were to entertain such a concern, the worry would not stop at the walls of the house, but would keep spreading:
From house to house.
From courtyard to courtyard.
From city to city.
And since there is no limit to such a concern - there would be no end to the matter, and therefore we do not take it into account at all.
In summary: once a person has finished his search, he may assume that the place has remained free of chametz. Only if he saw some sign that raises a suspicion that something occurred is there room for concern; but in the absence of any such sign, the search stands.