The first Mishna in Pesachim discusses the mitzvah derabbanan to do bedikas chameitz. Rashi writes that teh reason for the obligation of bedika is lest one violate “bal yera’eh u’bal yimatzey”, the prohibitions against ownsing chameitz. Tosfos disagrees, citing the gemara that even if one does bedika, one is still required to do bittul, declaring chameitz ownerless and like dust [bl”n more on how bittul works later], which effectively eliminates any possibility of violating bal yera’eh. The reason for bedika, says Tosfos, is not bal yera'eh, but lest one find chameitz and come to eat it during the chag.
Two issues that are consequential to the dispute: 1) whether one must do a bedika after Pesach if one forgot to do one before Pesach – there is no longer a risk of bal yera’eh, but there is still a risk of eating chameitz she-avar alav haPesach; 2) whether one must do a bedika for chameitz noksha, which according to many Rishonim cannot be eaten, but is not subject to bal yera’eh.
The Aruch haShulchan (633:12) writes that although the common practice is to do bedika in a shule or bais medrash, one should not recite a bracha or say bittul afterwards. If the reason for bedika is bal yera'eh, then there should in fact be no obligation of bedika at all as no one person is the formal owner of the shule and its chameitz; although we do bedika because of the reason of Tosfos, the bracha should not be recited m'safeik. Additionally, since only the owner of chameitz has the power to do bittul, the bittul of the gabai or shamash has no effect.
The gemara (4a) raises a question regarding someone who rents an apartment on 14 Nissan - does the obligation of bedika rest on the landlord who possesses the chameitz, or on the tenant who now possesses the dwelling? R’ Elchanan Wasserman points out that the gemara’s question may hinge on the dispute regarding the reason for bedika. If the reason for bedika is to avoid bal yera’eh, then the obligation of bedika should rest upon the owner of the chameitz. However, if the reason for bedika is lest one come to eat chameitz, then the obligation of bedika should rest upon the person using the dwelling, regardless of whether he is the formal owner of the chameitz or not.
What still needs explaining is why Rashi was concerned with the whole issue of bal yera’eh, if, as Tosfos points out, bittul removes the whole problem. Stay tuned…