particles - Differing explanations of 〜ところを

My textbook has this example sentence:

試験中、となりの人の答えを見ているところを先生に注意された。

The interpretation of this sentence hinges around ところを, and I'm not 100% sure what that means. The textbook simply indicates that it marks "while something is happening," and the ending particle (を in this case) matches whatever verb comes at the end.

If that's the case, then to me it looks like one of those "suffering passives" of the form "X suffered the teacher-warned-not-to-look-at-the-neighbour's-answers," where X is the subject. The gist of it being something like,

"During the test, we were warned by the teacher to not look at our neighbours' answers."

However, in my dictionary of particles it says that ところを "Indicates that something unexpected has happened or is happening."

That definition doesn't seem to fit in this case, because it would leave the passive construction in a weird spot...

But I'm still not sure how my textbook's explanation of "while something is happening" fits in, exactly. It made me think that maybe the translation should be something like,

"During the test, while I was looking at my neighbour's answers, I was warned by the teacher."

Which one of these is correct?

Answer

According to 明鏡国語辞典:

ところ 【所】
🈩〘名詞〙
⑥《連体修飾句を受け、時間的位置を規定する》 そのような場面・状況・事態・場合などの意。「家を出るところを見た」

The ところ in your example indicates the scene, situation, state, or occasion where something is happening or someone is doing something, and is often used in the form 「[人/物]が～～するところを/しているところを・・・」 for the English "SVOC" structure (like "I saw him crossing the street" "I heard her singing" etc.), eg:

佐藤さんは、私が授業中に居眠りしているところを見た。

Sato-san saw me nodding during class.
山田君は、私がその部屋から出ていくところを写真に撮った。
Yamada-kun photographed me leaving the room.

Turning them into Indirect Passive/間接受身:

（私は、）授業中に居眠りしているところを佐藤さんに見られた。
I was seen nodding during class by Sato-san.
（私は、）その部屋から出ていくところを山田君に写真に撮られた。
I was photographed leaving the room by Yamada-kun.

_{Their Direct Passive/直接受身 versions would be:
（私が）授業中に居眠りしているところが佐藤さんに見られた。
（私が）その部屋から出ていくところが山田君に見られた。
... but the Indirect Passive versions would sound more natural in most cases.}

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Now, your sentence:

試験中、（私は）となりの人の答えを見ているところを先生に注意された。

has the same structure as the two Indirect Passive examples above. Turning it back to active voice, you get:

試験中、先生は（私が）となりの人の答えを見ているところを注意した。
Literally: The teacher warned me looking at my neighbor's answers during the test.

_{Its Direct Passive version would be:
試験中、（私が）となりの人の答えを見ているところが先生に注意された。
... but this sounds pretty unnatural and uncommon.}

注意する in this context means "warn against" or "warn/tell not to do", so your example practically means:

"During the test, the teacher saw me looking at the answers of a student sitting next to me, and warned me not to do that. / told me to stop that."