On pages 66-7, Mawlana Manzur Nu'mani quotes a couple of hadiths to support the meaning of the narration "I do not know what is behind this wall of mine" as follows:
Originally Posted by Rahmaniyyah
And ignoring all those things, there is no doubt that the narration is true in its meaning, and many authentic hadiths support its content. For example, in the two Sahihs and Sunan al-Nasa’i, it is narrated from Zaynab, the wife of Ibn Mas‘ud (Allah be pleased with them), that in order to ask a question she wanted to ask regarding Zakat, she came to the door of the Messenger (Allah bless him and grant him peace), and when she reached there, another Ansari wife was standing there with the same need. Then Hazrat Bilal (Allah be pleased with him) came to them and she said to him: “Go to the Messenger of Allah (Allah bless him and grant him peace) and inform him that two women are at the door asking you: ‘Is charity permissible on their behalf for their husbands and for the orphans in their care,’ and don’t inform him who we are.” So Bilal asked him, and the Messenger of Allah (Allah bless him and grant him peace) said to him “Who are they?” he said: “A woman from the Ansar and Zaynab.” He said to him “Which Zaynab?” He said: “The wife of ‘Abd Allah [ibn Mas‘ud].” He said: “For them are two rewards: the reward of [maintaining good] relations, and the reward of charity.”
Thus, if the Prophet (Allah bless him and grant him peace) knew all matters behind a wall, what was the need for him to ask Hazrat Bilal (Allah be pleased with him) about the names? Then, after inquiring about their names and knowing that it is Zaynab, he asked “which Zaynab?” This is clear proof that he did not know some matters behind a wall.
Furthermore, in the last days of his pure life in the state of illness, in order to see his congregation, the Prophet (Allah bless him and grant him peace) went to the door of his blessed chamber, and opening the curtain, he saw those praying in congregation in the Prophetic Mosque - which is mentioned in the authentic books. Specifically during the final days he repeatedly asked: “Are the people praying?” Yet between the blessed mosque and the noble chamber was only one wall. This is clear proof that the Prophet (Allah bless him and grant him peace) did not know some matters behind a wall. Thus, if it is narrated in any hadith, “By Allah, I do not know what is behind this wall of mine” or as he (upon him blessing and peace) said, what is so farfetched and repulsive about it? Rather, nobody can dare deny the correctness of the meaning of this narration.
There is another narration I came across from the Sahihs of Bukhari and Muslim which demonstrates the meaning of this narration more clearly:
Sahl ibn Sa'd narrates:
اطلع رجل من جحر في حجرة النبي - صلى الله عليه وسلم - ومع النبي - صلى الله عليه وسلم - مدرى يحك به رأسه فقال : لو أعلم أنك تنظرني لطعنت به في عينك ، إنما جعل الاستئذان من أجل النظر
"A man peaked through a hole in the lodging of the Prophet (Allah bless him and grant him peace) and the Prophet had a comb with which he was scratching his head. Later [when he realised what the man did], he said: 'Had I known that you were looking, I would have jabbed it [i.e. the comb] into your eye! Asking permission [to enter a house] was only prescribed for the purpose of looking.'"
This hadith almost explicitly holds the same meaning as the narration: "I do not know what is behind this wall of mine," as the Prophet (Allah bless him and grant him peace) stated that had he known he would have jabbed this man's eye, and since he did not do so, it follows that he did not know.
Sahl ibn Sa'd was one of the younger Sahabah, so this event probably happened late in the lifetime of the Prophet (Allah bless him and grant him peace).