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Your sentence implies that action took place which the reader is not seeing (the "other means"). Wherever that sentence is, that's where/when you're placing this off-screen action. However, becau...
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#4: Attribution notice removed
Source: https://writers.stackexchange.com/a/10864 License name: CC BY-SA 3.0 License URL: https://creativecommons.org/licenses/by-sa/3.0/
#3: Attribution notice added
Source: https://writers.stackexchange.com/a/10864 License name: CC BY-SA 3.0 License URL: https://creativecommons.org/licenses/by-sa/3.0/
#2: Initial revision
Your sentence implies that action took place which the reader is not seeing (the "other means"). Wherever that sentence is, that's where/when you're placing this off-screen action. However, because the action of finding the letter physically happens in the next paragraph (that is, it's _not_ off-screen), it has to be the idea or the decision to search her backpack which is the Other Means in that sentence. - In the first example, the Other Means sounds like an idea which occurs to the narrator while s/he's questioning the girl. - In the second example, the Other Means is being presented as a narrative device: this is the narrator explaining something to the reader as though we were in the room with him/her, having an actual conversation about these events. - In the third example, it sounds like the narrator only came up with the idea to search the girl's backpack after she'd fallen asleep. Which one works better is simply a matter of when you want the idea to occur to the narrator.