Maachalos Assuros 13:1

Let’s say that a Jew buys or rents a building in a non-Jew’s courtyard and he fills it with wine. If the Jew lives in that courtyard, the wine remains permitted even if the entrance is open because the non-Jew will always consider the possibility of the Jew’s suddenly appearance. If the Jew lives in a different courtyard, he shouldn’t leave for home without first closing the building, holding on to the key and the seal. He need not be concerned that the non-Jew might copy his key.

Maachalos Assuros 13:2

If the Jew left without closing the entrance, or if he closed it but he gave the key to the non-Jew, it is prohibited to drink the wine because the non-Jew might have come in and poured a libation while the Jew was away. If the Jew told the non-Jew to hold his key until he returns, the wine remains permitted because he didn’t have him watch his building, he only had him watch the key.