How do you find a sentence?
If I want to look for a sentence or an old discussion I'll put the sentence into the discussion search box. Sometimes it will show up, other times it will show 0 results even if I only answered the question five minutes previously. It won't even show up on google search, or even under site search. Why is this? Where are you hiding them?
Sorry. What I meant was: Are you sure that anyone has ever commented on the sentence? Sentences without comments do not appear in the discussion area (unless someone comments and then deletes the comment later) and are therefore untraceable. I see I'm basically repeating stecchetto's answer and I'm sure it is correct.
Another way you can find a sentence is in the dictionary. Search for a word that's in the sentence. Maybe the sentence will pop up. Note that you can't access the sentence's discussion from there, if that's what you're looking for. However, sometimes you might need to look up the translation of the sentence, and in the aforementioned way you can achieve that.