Also here is how I would personally revamp the detection levels in future Bethesda games,
DANGER: Well obviously this means that at least one hostile enemy see you and is actively engaging in combat with you. Not much to be said here, it's' all hands on deck for them and they will bring out everything they got. As soon all enemies in the DANGER phase are unable to find you, you will enter the ALERT phase.
ALERT: In this mode, at least, one enemy does not have sight of you but is either fully aware of your presences or the presence of someone hostile to them or has previously seen you from the DANGER phase. This phase can also be triggered by companions, allies, friendlies that might be with you and even other enemies if you have more than one enemy faction in an area that aren't just hostile to you but to each other. There are many factors that could trigger Alert mode. Among these is the enemy has previously been in danger mode, the enemy has found a dead ally, the enemy has heard gun shots, explosions, people getting killed or whatnot. During this alert phase, the enemy will signal all of their allies (if there are any) of your presences or the presence of someone else hostile to them. They will do a full sweep of the whole area checking corners, behind or underneath furniture, wardrobes, containers, coolers, cabinets, haystacks and whatever the hell kind of places the player could hide in future games. They will also use smoke grenades, flashbangs, poison gas to draw out the enemy, fire at random places where they think the PC might be, lay down traps, puff up on chems if they are chem users.
In TES. mages will use detect life, cast candlelight spells all over the place as much as they can, cast armor spells like stone flesh, cast rune traps in areas, cast wards spells facing the direction they think the PC might be, cast shock spells at water if they think the PC might be hiding there, cast fire spells at haystacks or any flammable objects if they think PC is hiding there and etc. Archers/rogues will find cover and closely observe and either try to out sneak the player and get the drop on them in the case of the rogue or fire arrows in at where they think the target might be in the case of the archer. Warriors will raise their shields if they have one. If there is more than one warrior they might guard each others rear as they move around and depending on the personality of the warrior they might smash through doors and hiding places with their weapons and such. Everyone will drink boosting potions if they have any. I sure there are tons of things people can add to the list of things NPCS do during the ALERT phase, but we'll not waste any more time.
Note that if you entered the ALERT phase from the DANGER phase, it will never decrease so long as you are in the area. The simple fact is unless a person is a complete nincompoop with no sense of self-preservation they will not just slowly give up searching for an enemy they already saw earlier after only a few moments and pretend like nothing has happened. If you saw a thief breaking into your house and you cannot find him, are you are just going to give up and act like nothing happened? Of course not!.
If you entered the ALERT phase from the DANGER phase, it will only be a matter of time before the enemies find you and trigger the DANGER phase again unless you kill/pacify them before they find you or leave the area. When you leave the area you have to wait a while before things finally return to normal. The OP suggest 3-6 hours, but I will suggest 24 hours. Even if you are the one who triggers an ALERT phase, so long as you never triggered the DANGER phase, if a companion, ally, friendly, or opposing hostile is killed by the ALERTED enemy or found dead by them then the ALERT phase will decrease since the enemy would attribute the ALERT to that particular person who is now dead assuming you don't do something to stir up the phase again. Enemies in ALERT phase will not suffer from sneak damage bonuses.
CAUTION: At least one hostile enemy is suspicious but unaware of the actual presences of any enemies. Again this like ALERT phases is not just triggered by the PC but also companions and allies, even potentially other enemies if you have more than one faction in a location is hostile not just to you but to each other. This is triggered by noises, sight, and (hopefully in future games) smell. The sound is self-explanatory and you have my previous post to explain some of it. I'm hoping in future games, our character can emanate a sense of smell towards NPCS, so we could either smell like crap as a result of doing dirty things like swimming through a sewer or smell incredibly good if we say spray alchemical ingredients that smell good or perfume. Such smells if strong enough can also attract the attention of enemies. As for sight, if the enemy gets a tiny glimpse of the player for less than a second, that could also potentially raise suspicion causing them to go to CAUTION. Distance also plays a huge factor in smell and sight too. In caution mode, enemies will go and check out the source of the suspicion and will equip night vision, thermal vision, flares, torches, candlelight spells and whatnot if they are in a dark area and casually search for a few moments, if the enemy has friends nearby they might ask them to come with them to search. After a while, the detection meter slowly decreases from CAUTION to DETECTED or UNDETECTED. Enemies in CAUTION phase will still suffer sneak damage bonuses. Each time a CAUTION phase is triggered without it entering ALERT or DANGER, the amount of time it takes for CAUTION to decrease to DETECTED or UNDETECTED increases dramatically.
DETECTED: Detected by at least one non-hostile.
UNDETECTED: Well self-explanatory. There will be two different forms of undetected and I will explain which is which in a bit. All hostiles and non-hostiles alike both suffer from sneak damage bonuses.
Honestly, I find the stealth UI in both Fallout and TES to be way too in your face. For me, what I would do is make the detection meter a small orb at the bottom corner of the screen. It will grow brighter the closer it gets to the next alert phase and grow dimmer when it is closer to returning to the previous stage.
Green for completely undetected, blue for undetected by everyone except companions, white for detected by non-hostiles, yellow for caution, orange for alert and finally red for danger.
I'm kinda of iffy as to whether it would be good to just add 6 orbs representing each phase at the bottom with numbers on each indicating how many hostiles or non-hostiles are in each phase.