(I'll edit this as I find stuff)
What you generally are looking for then is 'Mystery NTR' although it's known by many different names.
https://f95zone.to/threads/summer-sweepers-v8b-impy.85528/ - You are a trio of adventurers made up of yourself (Percy), your childhood friend Fio and a tsundere big booty elf called Espa. On the island you do missions in the format of a minesweeper game but as you progress there are rivals for the girls affections and things can happen to them without the MC knowing. Also features 2 other routes where you twist the tables by becoming a pretty trap to lure the rivals away OR find out and decide to join in with the rivals.
https://f95zone.to/threads/lust-city-v0-9-premium-aid.13712/ - You are killed and come back to life a a futa/dickgirl, you decide to corrupt and have sex with lots of girls, standard fare. For you though there a few women that have love interests so you cuck them. Including one scene where the husband is sleeping next to you doing his wife. The game is translated from Russian and version 1.2 is currently being translated but work is slow due to unforeseen circumstances. Still good to play though. You WILL need the walkthrough.
https://f95zone.to/threads/dickgirl-dorm-development-thread.71958/#post-4956939 - Best game hands down (I am not biased at all in any way). But joking aside if you can stomach futa/dickgirls there are a few scenes in the Netorase route (No netorare yet but my next update my patrons voted for it to be put in) where although you've given permission you can't really see what is happening (the MC, the player can of course) so for example one scene has one of the characters talk to you while your wife is on all fours behind the counter being plowed but the MC can't see that. It's still early days and looks old, I'll be the first to admit I'm working more and more on it. But it's a side project and I'm surprised how many people have liked it.
https://f95zone.to/threads/the-adventurous-couple-ch-14-monday-mircom3d.7192/ - name says it all. On the whole it's a more swinging or Netorase game where a couple spice up their sex life with some swinging, some voyeur. Both partners can get some BUT there are also possibilities for both the MC and the wife to do it secretly and cheat as well.
https://f95zone.to/threads/ntr-homestay-final-anim-teammm.75702/ Decent VN with multiple perspectives as the story goes along. Typical Japanese husband MC who host an exchange student as his home, said student is a 7 foot blonde bodybuilder. You can play from the persepctives of the MC, the wife and the daughter. MC route is pretty oblivious until the end. Also features quite a build up too. But there are scene where things are happening during phone calls and other sneaky stuff.
https://f95zone.to/threads/pervert-action-timelapse-v0-27-3-bbben.25638/ A game where you woo the affections of 3 women, but you can also pimp them out to earn money. However if you keep doing that eventually they will sneak around and do stuff without the MC knowing and only finding out afterwards via camera.
https://f95zone.to/threads/ema-milf-warrior-ruins-of-the-ancient-empire-heisendou.621/ The CLASSIC. One of the oldest there is. Play as a female adventurer with her mage husband, you can do missions together or go off on your own. Sneaky scenes are things like sleeping with the innkeeper to pay rent or with the guy in the room next door while the husband sleeps.
https://f95zone.to/threads/president-yukino-v1-09-01-final-acerola.19125/ Female MC that gradually becomes corrupt and sneaks around cheating on her boyfriend, If I recall correctly he can find out or remain oblivious unless I'm thinking of another game similiar.
https://f95zone.to/threads/an-obedient-childhood-friend-is-easily-cucked-final-udagame.28121/ - Old school black and white, you can start to pimp your girl out and eventually she will start doing it behind your back sneakily.
https://f95zone.to/threads/raiohgar-asuka-and-the-king-of-steel-v1-04-gaptax-kagura-games.52355/ - Decent game from the perspective of the boyfriend and girlfriend. Playing as the boyfriend will occasionally have sex sprites in another room of someone doing your girl sneakily, and the towns folk will keep hinting more and more about your girls experiences.
https://f95zone.to/threads/scars-of-summer-v1-03-shinachiku-castella-kagura-games.55259/ - You have a girl you are interested in, but you also have rivals too. If they get enough hearts from her then they will start doing stuff with the MC remaining oblivious
https://f95zone.to/threads/the-edge-of-v1-0-hangover-cat-purrroduction.71672/ A game where a guy suspects his girl is cheating and delves into even more deeper paranoia so starts using cameras etc. If I remember this was the one where a lot of people actually wanted the girl to get away from the MC.
https://f95zone.to/threads/building-our-futature-v0-66-4-infidelisoft.44841/ If you can stomach futa/dickgirl this one is good, you are recently hired by your boss Dan, you make friends with his wife during dinner to get to the know the boss. From then on you keep popping round and corrupting the wife until you get your dick wet. sneaky stuff are getting a BJ while Dan is on the phone, having sex in a closet while he is in that room etc etc, you also sometimes have the dialogue after scenes with the husband and wife. Very sneaky type game.
https://f95zone.to/threads/holding-hands-v0-09-kirantiplayer.88368/ - You and your girl have a lodger come to the house, you can go down multiple routes and paths and some of them do feature sneaky scenes.
https://f95zone.to/threads/secret-care-cafe-v0-7-2-public-rare-alex.63911/ More netorase but some of the scenes take place away from the MC and he is not there to know what is happening.
https://f95zone.to/threads/sana-v1-5a-breast-mafia.25644/ Huge tits, be warned. You recently moved to village/town and gradually everyone wants to fuck your wife. sneaky scenes include sneaky in battle sex, her arse being groped when you're not looking and sex scenes around town, that the MC remains oblivious to.
https://f95zone.to/threads/ntr-adventurer-alisa-mtl-final-rain-shelter-frog.70287/ - Didn't play this too much but the main picture shows the typical sneaky stuff, MC reading a book while girl railed next door.
https://f95zone.to/threads/ntr-knig...le-are-being-fucked-v1-3-bitch-bokujou.35651/
https://f95zone.to/threads/okaken-no-hihou-kyoufu-no-ntr-sugoroku-v1-03-moccori-factory.79125/ - A...board game, that features NTR if you stray too far from your girl.
https://f95zone.to/threads/the-ruins-of-luxoria-final-mijiishi.48387/ Netorase to begin with, i.e. permission given since the MC and the girl share a mystical link where if she takes in semen, they both get stronger, so at first she i reluctant, then you pimp her out and eventually she can be found doing stuff on her own around town.
https://f95zone.to/threads/cucking-trap-v1-0-5-double-melon.91244/
https://f95zone.to/threads/legender...eroes-tormented-v1-02-ankoku-marimokan.82329/ play as a male and female adventurer team, stuff can happen sneakily to either one. Indeed one of the endings is the boyfriend being stolen from the girl by another guy.
https://f95zone.to/threads/teaser-date-kanon-v1-0-fullbluffstudio.68791/ - Very good for sneaky stuff, it doesn't show the MC any of the cheating sex but what it does do is you hear the sounds, read the dialogue, the build up to your girl doing sneaky stuff.
https://f95zone.to/threads/the-world-with-no-memory-v1-0-fluffylynx.65863/ - Has multiple routes but one of them is she cheats without her guy knowing.
https://f95zone.to/threads/the-queen-who-adopted-a-goblin-v1-1-ntrman.68270/
https://f95zone.to/threads/the-fate-of-choice-v1-0-broken-english.53591/
That should be a small sample to whet your appetite. Enjoy.
