What are the nistaros, the hidden sins?

The famous pasuk in Nitzavim reads:

The hidden things belong to the Lord, our God, but the revealed things apply to us and to our children forever: that we must fulfill all the words of this Torah.

כח. הַנִּסְתָּרֹת לַיהֹוָה אֱלֹהֵינוּ נקודות) וְהַנִּגְלֹת ֹלָֹנוֹּ ֹוֹּלְֹבָֹנֵֹיֹנֹוּ עַד עוֹלָם לַעֲשׂוֹת אֶת כָּל דִּבְרֵי הַתּוֹרָה הַזֹּאת:

But what are these hidden things?

According to Rashi, this is a response to the context above, in pasuk 17-20, which indicated punishment for an individual's thoughts [that his heart will turn to follow other gods, and that on hearing the words of this oath, will reassure himself], followed by pesukim 21-27 describing drastic communal punishment. Therefore, this pasuk comes to clarify. In Rashi's words:

The hidden things belong to the Lord, our God: Now, you might object [to God, saying]: “But what can we do? You punish the entire community because of the sinful thoughts of an individual, as Scripture says, ‘Perhaps there is among you a man…’ (verse 17 above), and after this, Scripture continues, ‘Seeing the plagues of that land [and the diseases with which the Lord struck it]’ (verse 21) [which seems to indicate that for the sinful thought of even one individual, the whole land would be struck down with plagues and diseases]. But surely no man can know the secret thoughts of his fellow [that we could somehow prevent this collective punishment!” In answer to this, God says:] “I will not punish you for the hidden things!” [I.e.,] because “[The hidden things] belong to the Lord, our God,” and He will exact punishment upon that particular individual [who sins in secret]. However, “the revealed things apply to us and to our children” [that is, we are responsible for detecting the sins committed openly in our community, and] to eradicate any evil among us. And if we do not execute judgment upon these [open transgressions, over which we do have control,], then the whole community will be punished [because they would be remiss in their responsibility]. There is a dot placed over [each letter of] the words לָנוּ וּלְבָנֵינוּ here, to teach us homiletically that even for open sins [which were not brought to judgment, God] did not punish the whole community-until Israel crossed the Jordan. For then, they accepted upon themselves the oath at Mount Gerizim and Mount Ebal, and thereby [formally] became responsible for one another (Sanh. 43b). [When dots are placed over letters of the Torah, this denotes an exclusion of some sort. In our context, our Rabbis teach us that the exclusion refers to the period prior to the crossing of the Jordan.]

הנסתרת לה' אלהינו: ואם תאמרו מה בידינו לעשות, אתה מעניש את הרבים על הרהורי היחיד, שנאמר (פסוק יז) פן יש בכם איש וגו', ואחר כך (פסוק כא) וראו את מכות הארץ ההיא, והלא אין אדם יודע טמונותיו של חבירו, אין אני מעניש אתכם על הנסתרות, שהן לה' אלהינו והוא יפרע מאותו יחיד, אבל הנגלות, לנו ולבנינו לבער הרע מקרבנו, ואם לא נעשה דין בהם יענשו את הרבים. נקוד על לנו ולבנינו, לדרוש, שאף על הנגלות לא ענש את הרבים עד שעברו את הירדן משקבלו עליהם את השבועה בהר גרזים ובהר עיבל ונעשו ערבים זה לזה

Rashbam links the nistaros to those he described earlier by the klalot and brachot.

פסוק כח 
הנסתרות לה' אלהינו - כבר פירשתים אצל הארורים על הנסתרות (כי) הנסתרים היו הברכות והקללות, שאין הדבר ליענש ביד בית דין אלא ביד הקב"ה. 

That is, in Ki Tavo. The sins there, as described by Ibn Ezra, Rashbam, and others, are ones committed privately, and Hashem takes care of the punishment. (Thus, they are cursed.) See Rashbam there. Those include a variety of sins.

Ibn Ezra keeps it closer to the overt context, private idolatry:
הנסתרות -הטעם: מי שיעבוד עבודת כוכבים בסתר. 

לה' אלהינו -והטעם: כי משפטו ביד השם והוא יפרע ממנו, ואם היתה בגלוי חיוב לנו ולבנינו לעשות ככתוב בתורה. 

Ramban argues with "meforshim" [presumably Rashi and certainly Ibn Ezra] for saying this refers to private sins of idolatry. Rather, he asserts, it refers to accidental sins. Is he saying that the person is unaware of these accidental sins? I am not sure. Further, he says that Onkelos holds like him:

הנסתרות לה' אלוהינו - על דעת המפרשים: יאמר כי השם אלוהינו לו לעשות משפט בעובדי עבודה זרה בסתר, כי כל התעלומות גלויות לפניו, והנגלות עלינו ועל בנינו לעשות להם את כל דברי התורה הזאת, להכות עובדי עובדה זרה כדין התורה. וגם כפי המדרש (סנהדרין מג ב): כן הוא:

ודעתי בדרך הפשט, כי "הנסתרות" הם החטאים הנסתרים מן העושים אותם, כמו שגיאות מי יבין מנסתרות נקני (תהלים יט יג), יאמר הנסתרות לשם לבדו הם אין לנו בהן עוון אשר חטא, אבל הנגלות שהם הזדונות, לנו ולבנינו עד עולם לעשות את כל דברי התורה הזאת חוקת עולם, שכך קבלנו על אשר ישנו פה ועל אשר איננו פה לדורות עולם. ולפי שהביא באלה לעשות כל המצווה, הוציא מן החרם העושה בשגגה שלא יתקלל באלה הזאת.
ודברי אונקלוס מטין כן, שאמר: 

דמטמרן קדם ה' אלהנא.

ואם כדברי המפרשים ראוי לו לומר דמטמרין לה' אלהנא.

I admit that I am unclear about the Ramban's diyuk in Onkelos here. Onkelos said:

כט,כח הַנִּסְתָּרֹת--לַיהוָה, אֱלֹהֵינוּ; וְהַנִּגְלֹת לָנוּ וּלְבָנֵינוּ, עַד-עוֹלָם--לַעֲשׂוֹת, אֶת-כָּל-דִּבְרֵי הַתּוֹרָה הַזֹּאת.  {ס}

דְּמִטַּמְרָן--קֳדָם יְיָ, אֱלָהַנָא; וּדְגַלְיָן לַנָא וְלִבְנַנָא, עַד עָלְמָא--לְמֶעֱבַד, יָת כָּל פִּתְגָמֵי אוֹרָיְתָא הָדָא.  {ס}

Is the diyuk based on the דְּמִטַּמְרָן rather than דְּמִטַּמְרִין ? Or is is the קֳדָם יְיָ rather than ליְיָ? Or both? I think the difference in spelling, which is just a masculine vs. feminine distinction, is incidental, perhaps just a scribal error in the copyists of the Ramban. But the focus is that they are kadam Hashem, before Hashem, as a change from the Biblical Hebrew laHashem. With this change, the meaning is: not before us, such that we need not worry about them. Rather than them being la-Hashem -- for Hashem to punish.

I am not convinced by this diyuk in Onkelos, because the use of kadam as it relates to Hashem is a regular anti-anthropomorphic feature, or a distancing from direct ascription to Hashem, as a mark of kavod.

[Netina Lager mentions this diyuk but does not explain it.]

A final word: Nowadays, should we be focused on the private wrongdoing of others? Yes and no. I am not in favor, in general, of inquiring into the private religiosity of others. E.g. in the hashkafic domain -- so long as people don't go out and make a website to convince others of their views, there is no reason to inquire and cast aspersions on the religious beliefs of some of our more left-wing brethren. Or maybe even to judge them for their beliefs. Bli neder, more on this in another post. Or schools which hold Internet is assur, and take pains to control the private behavior of parents.

On the other hand, many of the sins Rashbam referred to as secret and private sins, in Ki Tavo, had a victim. Someone was secretly perverting justice via bribery, and causing the widow and orphan to suffer. Or cases of incest, where the female victims were not in a position of power, and thus could not make it public. (See Ibn Ezra on these pesukim.) Should we indeed throw up our hands and say that it is up to Hashem to take care of, and we can only take care of the niglot? Well, at the least, when it becomes revealed to us, it is upon us to act and defend the powerless.