I am responsible for an apartment in Jerusalem, owned by a family member, which is rented out for short periods, ranging from a night to over a month. In general it is rented to pretty much anyone. When it is rented out, it and all it's contents are the responsibility of the tenant.
Does it matter (to me or the owner) if the person I rent it to during Passover brings chametz into the property:
Pesahim 4a, 4b and 5b discuss thoroughly this topic.
For example, my English translation of Koren Talmud Bavli cites the halacha
If the owner of a house who lets it to another delivered the keys to the lessee before the evening of the fourteenth of Nisan, the lessee is obligated to search for leaven. If the keys were delivered afterward, the lessor is obligated to perform the search (Shulhan Arukh, Orah Hayyim 437:1).
It says also
One violates the prohibition against having leaven seen or found in his possession for any leaven that belongs to him, even if it is not in his home or it is in the house of a gentile. However, one is not liable for leaven that belongs to a gentile or which has been consecrated, even if it is in his house. This is the case even if the Jew overcame the gentile or if he is a resident alien, in accordance with Rava (Rambam Sefer Zemanim, Hilkhot Hametz UMatza 4:1–2).
I think the latter applies to the owner of the house you're taking care of. The former also speaks about the owner. I don't know if it also applies to the case when there is an intermediate stage (you) between the lesser and the lessee, however I think it does by transitivity argument and the owner actually delivers the keys to the lessee.
