Where is the boundary for checking for chometz, must one check the whole world for chametz, perhaps he has left chometz their?
There are two parts to getting rid of our chometz. The first is that we shouldn’t own any chometz, and for this we nullify ownership and make any chometz that we might have, into ownerless and hefker. Although technically we can fulfill the biblical commandment by nullifying the chometz, we are also commanded by chazal to physically search and rid ourselves from any chometz that may be in our property. This is because we may come to eat it, if it is in our possession. Therefore if the chometz is out in the street, and not in our property, we do not have to get rid of it.
Rambam Hil. Chometz U’matza 2-2,3.