Instead of using roll under Dexterity, a Stealth challenge roll, or a Stealth-As-Combat system for determining sneaking- use a Ruling. I was considering calling this "Dirt Simple Stealth", but this is a bit too complicated and abstract for that. Instead, consider it a meta commentary on rules.
There are three categories in ways you can be detected. They are sight, sound, and smell.
Sight is the most important for humanoid enemies (who are tall and see over brush) and is the hardest to mask when outside.
Sound is the most important for monster enemies (live in darkness) and is the hardest to hide when you are close by.
Smell is the most important for animal enemies, beast people, and freaky humanoids (like Wizards) and is the hardest to hide indoors.
In order to mask yourself in one or more categories, you need to prepare a method to hide one, two, or all three of these factors. Sight is hidden by darkness and camouflage, as well as moving around corners and staying still if directly observed. Sound is hidden by not moving, as well as wearing proper shoes and watching what you step on. Smell is hidden by taking a bath and wearing clean clothes, but cannot be fully hidden and leaves a trail. Smell also builds up in places over time- so the longer you stay in one spot, the more likely your smell will build.
Using the above rules, let's go over a few stealth scenarios.
Scenario #1 – Orcs at a Campfire
Your party are traveling through the woods at night when they come across a group of orcs around a camp fire. Most of the orcs are asleep, the few who are left on watch are sitting around a fire. They are clearly staring into it and their eyes have adjusted to the light; not to the darkness around the camp. Orcs are considered humanoid (two legged), monstrous, AND they are often depicted with animal body parts- so one could say they are quite difficult to sneak past.
Because it is night time and there is plentiful bush, as well as the facts the orcs are staring into a fire instead of the darkness you are not going to be spotted. Because you're outside and because there is also a fire- your scent is also masked. As such, your DM tells you the only way for you to get detected is if you make too much noise.
The intrepid Rogue says he'll sneak around the orcs silently. Perhaps because he has a high Dexterity score, or perhaps because it is an innate ability of the Rogue class, he can move silently. The DM says that the player may step on a branch, so the player says he'll take his time- a full turn to move past these orcs and clear the path for his party. The DM ticks the time off their time tracker and lets them go through without incident. In this situation, it was relatively easy to get past and the only thing expended was time.
Scenario #2 – Guards at the Gate
Your party got kicked out of town last week and now has to sneak back in to meet with an important NPC. There are two guards standing by the gate in the stone wall. It is in the middle of the day and as such the guards will see you coming- your faces are known from wanted posters and infamy, so you'd be easy to spot. Secondly, there's no way to sneak around them, but the guards are wearing heavy metal helmets and are only humans- so sound and smell are less important here. The guard visually check everyone who comes past.
Your party comes up with a plan. They take on disguises as praying monks and hire a villager to speak for them. “They've taken a vow of silence-” the villager says, as you pass him a silver for his service and they let you into the gate, far too busy with inspecting the incoming merchants for contraband then to bother a group of penniless holy men. In this case, you bypassed the stealth test by expending a resource- wealth or a contact.
Scenario #3 – Dragon Hoard
The party comes across the giant dragon sleeping on a hoard of gold and jewels. There is one specific item you need; the Sacred Sapphire. You'll have to steal it from the sleeping dragon. The area is in its cavernous lair and the air is thick with dragon musk- with no winds or other smells (that the dragon isn't used to) your smell will be a big giveaway. Plus dragons are considered monsterous and animal- but not humanoid. Especially since its sleeping- sight isn't a concern.
Your party ducks away to an earlier place in the dungeon with a water fountain and showers, changing into fresh linens. Then, you take some of the fine ladies clothes you found earlier in the dungeon and tie them around your feet to act as soft pads for walking. The dungeon master tells you that your weight could cause an avalanche of coins- so you only send your smallest and least encumbered member up the hill of gold to grab the Sacred Sapphire.
Your DM tells you your plan works- the dragon nostrils do not flex to smell the invaders and your feetfalls make no noise as you approach. But the moment the halfling picks up the gem, the dragon begins to stir- its supernatural sense for its treasure hoard wakes it up. Its eyes open and locks on to the party, who otherwise stick out like sore thumbs among the piles of shiny gold.
In this case- you did everything right, but not all opponents can be countered by traditional methods of stealth. In this case, you would have need more legend lore to know about the dragon's abilities to sense its hoard, cast a magic spell of invisibility, or perhaps used your stealth round to set up an advantageous first round of combat instead of getting away scot-free. Maybe a decoy gem of equal value put in its place may have soothed the dragon enough for it to leave you be- at least until it wakes up a few hours later later from the constant disturbance of the new jewel slowly lulling it to wakefulness, much like an irritating bug crawling on your skin takes time for you to become fully aware of its presence.