How do the regular monsters "see" in Amnesia?

Here are a few observations:

• Their vision is heavily based on light/darkness. Early in the game it will tell you to turn your lantern off to avoid them. Standing in ambient light isn't quite as bad, but still results in visibility. This can sometimes result in a decision to be made regarding being seen vs. losing sanity.
• They seem to have basic hearing. It's usually the least you need to worry about, as most anything you can do that makes noise will also result in increased visibility. But if you're moving around right behind them, they'll notice.
• Proximity seems to be a huge factor. If they're really far away, they're unlikely to see you until they get closer. You can often see them before they see you, depending on the lighting in the area.
• If one is chasing you, run until you can get both: a good distance away, and around a corner or. Once you've broken the line of sight, they will only follow you for what seems like a predetermined distance, which you'll probably get a feel for. Might as well turn your lantern on while you run.
• Looking at one will slowly drain your sanity, but it can be useful once or twice to get a feel for their movement speed and patterns.
• If one hasn't seen you yet, turning off your lantern and crouching is usually more than enough to get you to a safer hiding spot without being spotted. I find crouching behind open doors to be very effective.

I apologize for giving an unsourced answer, but I don't really expect there to be any "official" answers out there. Hopefully some more people can come along and give you a consensus.

It's pretty simple. I'm a map maker, so I've been playing with them.

I'm pretty sure it is actually possible to sneak behind them while crouching, but touching them will alert them.

The grunt reacts to sound! I think walking will not alert them, but they hear you run, so don't run when you are near them. Throwing a box is also alerting them. If the grunt suddenly make a loud groan, then it means it has heard something. It will walk towards where the sound was.

Monsters are very bad at corners, so run close to the corners, jump over tables and stuff. This will give you time to find cover behind something.

Crouching is always a good thing!

I've been playing amnesia for a while now and I think I understand the monster behaviours fairly well now.

The most important thing you need to know is (as a rule of thumb) to hide. :D

Avoiding detection consists of 3 parts: 1. Reduce your sound (crouching helps) 2. Reduce visibility 3. Increase your distance

If the enemy sees you going in a room or corner, it is only bound to follow you to some extent. If there is nowhere in the room to hide, (such as no closets) they won't come in the room and will just stand at the entrance for a while. (at least that's what happened to me.)Also, they will not travel long distances to chase you. After you either run a certain distance away or hide for long enough, they will de-spawn so, not to worry.

So therefore, if you pretty much break visibility between you and the monster and are able to stay for about 10 seconds, then you're safe.

The developers made the monster fairly easy to evade as to not excessively frighten the gamers and avoid more "effect" than necessary. :P

All of the stated above were based on my own experience and gameplay. So, don't underestimate them; you should still be extremely cautious.

Have fun!