Is unlocked enough? Doesn't it need to be slightly open, so that there is a concern that someone might come in / see?

The Rashba (Responsa, vol. 1, no. 1251) wrote that merely closing doors does not constitute seclusion unless the house is locked. And so too wrote Rabbenu Yonah (Sefer HaYirah, ad loc.): "Do not be alone in seclusion with any woman, etc.—and what is considered seclusion? Closed with a key or with a lock, as in the law of seclusion stated in the Torah." And many leading halakhic decisors wrote likewise (Responsa Radbaz, vol. 1, no. 121; Mabit, vol. 1, no. 287; Responsa Maharam Schick? no—Maharshal? Actually מהרש"ם: Responsa Maharsham, vol. 2, no. 76; and others). It is also reported in the name of Rabbi Avraham Yeshaya Karelitz, the Chazon Ish (Sefer Devar Halakha, no. 3, sec. 6), that an apartment on a high floor whose door is closed but not locked, and where people may pass through the stairwell, is considered an open doorway.