It is forbidden to steal from a non-Jew. However once one has stolen, is there a mitzva of veheishiv es hagezelah (returning the stolen property), and is there a difference before yiush or after?
As you note, it is forbidden to steal from a non-Jew, and if a person did steal from a non-Jew, he has an obligation to return the stolen property. This emerges from the reading of the Rambam and the Shulchan Aruch (Choshen Mishpat 359-360), who introduce that the prohibition of stealing applies even to stealing from a non-Jew, and proceed to explain that one who stole must return the lost property.
The Maharam Chaviv writes that the obligation to return stolen property is only rabbinic in nature. However, others maintain that there is a Torah obligation to return the stolen property. The dispute is noted by the Sha’ar Hamelech (Gezeilah).
The Sha’ar Hamelech (Gezeilah 1:2) also notes a dispute concerning the question of yiush. According to the Yom Teru’ah (6a), after the non-Jew is meya’esh (loses hope of getting back the stolen property) there is no longer an obligation to return it, and this also emerges from the Shiltei Giborim (Sukkah, beginning of Lulav Ha-Gazul). However, the Sha’ar Hamelech explains that other rishonim dispute this position.
For the reasons above, and also for reasons of potential chilul Hashem, one should of course be sure to return any stolen property to its owner, Jewish or otherwise.