Make the world continue while they are gone ;)
Let me explain, the world does not stop when the players are not there. For your example, I have more than one idea popping in my mind:
- Unless the cart was theirs, it should not stop for few adventurers running off. If it was theirs, it is left unsupervised. An unsupervised cart with provisions could be stolen, or ravaged by few wild animals looking for easy food.
- If they left a sentinel, the cart could be attacked by bandit and the sentinel held for a ransom.
- If the cart belongs to an NPC and continues without the players, it could be attacked by bandits :)
- You may also reward the player with negative consequences by setting an ambush, or missing an opportunity due to the wasted time, such as having been able to help a wounded man if they stayed on cart.
- The player could chase the smell to discover a dead bird fallen from a nest, a dog digging up a corpse (and the locals discover the players digging up the grave and accuse them of desecrating a grave or causing the death of the poor guy).
You may also hint to the players that the next safe resting point, like the inn is a long way away and unless they want to camp or drive the cart by night, they should stay on road.
In short don't reward the lunatic. If someone sees a shadow disappearing in a dark alley without reason, they could walk in bigger trouble than they can handle. But don't kill them for just one bad decision, make it clear that the villain is way too powerful, and they have to flee, and then hide for weeks, putting some spice in the intended scenario :)
Don't be an ass and punish them every time the stray from the path, try to be logical. Reward them sometimes, and make some situation where their decision is important (they have unburied half of the corpse when they hear 2 voices closing in. Do they hide? flee? try to explain?
