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.