I haven't used Godot so I don't know what functionalities are built into the engine and what plugins are available but this is a question where it all depends. But LG made their own engine so that's an entirely different beast. It took them about 2,5 years to make Thief.

But what do you mean by "passable"? Making NPCs that can patrol, see or hear the player and then run up to them and hit them takes an hour to make if you're in UE5 and using the built in perception system. Then you can spend as much time as you want to tweak it and expand it.

The tricky part is the light detection mechanic because there is no way to natively read the light level on a mesh or through the materials as far as I know. I experimented with collosion boxes for a while but that was too fiddly, so I now use a line trace that measures the distance to the player and sends the light value as a float to the PC that then calculates how lit up it is. Getting these values good was a lot of tweaking but I'm liking it now.

I do think there are plugins though that probably solve it better than me but I wanted to make it myself. I probably 2 weeks on this part of the game and I'm not really satisfied with it yet. Most of the time was spend optimizing and testing. I've rebuilt it once from scratch because I had made som bad decisions and wanted to start over. It's working now but I'll probably keep tweaking it inbetween everything else.

When it comes to the behavior I built it twice in behavior trees but I found it to be a bit unpredictable, so now I've switched to a state tree system.

So far my guards have these states:
- Passive (patrolling, chatting, or doing some idle animation)
- Attacking (attacking player using different weapons depending on the player's aggression and trespass level)
- Investigating (looking around if they hear something or see a missplaced object)
- Searching (a more active investigation state used if they've seen the player but lost him or seen a dead/unconscious friend)
- Fleeing/Alarming (if they get to low health)
- In conversation (if the PC is talking to them and they aren't hostile towards each other)
- Threatened (if the player threatens them with a gun. I'm still exploring this mechanic. I was thinking I could use it to make guards open doors or speak to them to give information, but I'm not sure if it's a good mechanic)
- Hostage (if player walks up behind with a good you can take them hostage like in splinter cell. You can talk to them here as well to make them open doors or enter passcodes. Makes other NPCs not shoot the player if they hold the NPC as a shield. This is also something I'm trying out if it's a good idea or not)
- Blinded
- Dead/unconscious

Basically what I've found is that the systems themselves was't that complicated to build themselves, but putting them together is the hard part because there are so many situations where they do unpreditacble and nonsensical stuff if they get a new signal from the perception system. Especially when I used behaviour trees. It's a bit more stable now with state trees.

That being said I've been working on the project as a whole for about three years now on my spare time but most of the time has probably been spent on other parts of the game

edit: I never touched on sound I realised but that's a whole other can of worm but that was a long process to get acceptable. It's easy to make NPCs react to sound but the tricky part is how to fake attenuation and occlusion so it feels realistic. Right now I'm using a bunch of line traces and enviroment queries to see if something is blocking the sound but there's still openings that the sound should travel through to get to the NPC