Well there is mystery and then there is mystery. One of those gives you nothing, just the girl starts being a bit distant etc.
The other ones has you asshole coworker as your neighbor, who regularly brings girls to fuck and is loud while doing it. But this time the voice sounds a bit familiar? Huh, why are you all ruffled while working? What is that white liquid? Where are your hands when you are sitting side by side with the douche coworker?
The second type is hot. It gives you SOMETHING that is happening just outside your vision. The first one is just plain outside your vision. Its boring and its imo pure crap. Honto no Kimochi is itself pretty good, just untranslated... you also arent forced to play mc only pov first.
I guess the latter type of mystery you explained could be expanded into the term (let's just say) "world-building". I too believe this tends to be only a sparingly utilized element in Netorare AVN storytelling specifically, which (to give the devs the benefit of the doubt) is likely due to logistical, resource, and/or time constraints. Because it may or may not provide just a marginal improvement to the main narrative, and with diminishing returns since the market of NTR works is not as massive as the vanilla works (where the expansive world-building would more promisingly pay off).
For example, netorare stories can benefit from making the story starts
before the male MC and the female Love Interest became an item. This way, the devs may have more room to build suspense via foreshadowing, by (for example) suggesting the female MC having slutty tendencies or past experiences. The dev can counterract this by establishing a solid reason for the female LI to fall in love with MC (which certainly is not the hardest thing to do), but the audience knows that this doesn't necessarily undo her past. So the element of suspense will still be there and ever-looming.
Another narrative device to help craft world-building is using side characters to establish an adulterous atmosphere, for example the female LI (now girlfriend) still keeping in touch with her "party girls" friend group. And the dev can then explicitly show those side characters' corruption (as in, being slutty), and it will positively contribute to the suspense in Male MC & his girl's storyline. The proximity with the party girls will evoke a sense of foreboding, as in the feeling that the girlfriend may likely fall into cheating because of the influences of her friends.
And of course there is always the narrative device of cyber-spying. Think snooping into private social media accounts ("finsta"), private messaging apps, remote access, hacking into surveillance cameras, etc. It doesn't necessarily have to involve the female LI (girlfriend) herself, at least in the first half of the story. But the dev should focus on establishing the cultural "feel" and atmosphere of the story's setting. For example just showing that the girlfriends' peers (both boys & girls) from campus is going house parties, yachting, raves, etc. And let the (once again) proximity do the work in building the narrative suspense. Ominously suggesting that the girlfriend may feel left out being "tied down" with MC, and so her subsequent outings ("girls night/trip) with her friends may entail something happening behind the curtains.
So the main point i'm trying to convey here is; the tension build-up doesn't have to always actively involve the female LI (girlfriend) herself. The "world" she chose to passively place herself in also greatly tells something about herself. And the reveal of these "tidbits" may be done gradually, so as to not make it seem like the girlfriend is (for some reason) forcing herself to be in compromising/risque situations all the time for the sake of building tension for the netorare plot.
Just giving my two cents. And by all means, let me know if i'm wrong in assuming these narratives techniques being rarely utilized. Because i'd
LOVE to play some games/VNs with these elements ASAP.
EDIT: o shit, just realized you're the same dude i chatted with some time ago here. What's good