All perfect praise be to Allah, The Lord of The Worlds, and may His peace and blessings be upon our Prophet Mohammad and upon all his family and companions.
If the witness is concealed in a place where he can neither be seen nor heard by the defendant, then his imposition is definitely valid because its conditions are met and it isn`t a condition that the defendant should know that the witness could see and hear him. It is also not conditioned that the defendant should approve of the witness`s attendance.
Ibn Najim(May Allah be pleased with him) said: "It isn`t conditioned that the defendant should know about the witness. Therefore, if the latter could see his face and understand what he is saying without the former knowing of his presence, then his testimony is acceptable."[Al-Bahar Al-Ra`eq, vol.7/pp.69].
Moreover, Ibn Hajar Al-Haithami (May Allah be pleased with him) said: "It is conditioned that the testimony should be heard and the witness should be seen, even if he was behind a screen" [Tohfat Al-Mohtaj, vol.10/pp.258]. And Allah Knows Best .