Sneaking

In most cases, sneaking requires an opposed roll. The sneaking character rolls Dexterity + Stealth (difficulty varies by availability of cover and how inherently noisy the ground is) resisted by the target’s Perception + Alertness (usually 6).
If the sneaker gets more successes than the guard, they remain undetected. If the watcher beats the sneaker’s successes the sneaker is detected outright. If the rolls tie the watcher is alerted to the sneaker but isn’t able to solidly identify what they look like.
A botch on the sneaker’s part means they do something obvious such as breaking a ceramic pot or stepping on a cat’s tail. A botch on the watcher’s part means he is completely inattentive to his surrounding, maybe engrossed deep in a conversation or even falling asleep. If it’s not practical to make an opposed roll against every potential spotter, a character can roll Dexterity + Stealth against a flat difficulty based on the overall security of the area with the degree of successes showing how well the character remains undetected.