Scent (5e Variant Rule)
From D&D Wiki
This extraordinary special quality allows a creature to detect approaching enemies, sniff out hidden foes, and track by sense of smell. Creatures with the scent sense can identify familiar odors just as humans do familiar sights. Creatures that would have this sense normally have Keen Smell trait or similar feature such as the Wolf, but it is ultimately up to the DM to grant this sense to a creature. The Scent sense will grant the creature the ability to do the following below:
The creature with the Scent sense can detect other creatures within 30 feet by sense of smell. If upwind, the range increases to 60 feet; if downwind, it drops to 15 feet.
Strong scents, such as smoke or rotting garbage, can be detected at twice the ranges noted above. Overpowering scents, such as skunk musk or troglodyte stench, can be detected at triple the ranges noted above.
When a creature detects a scent, the exact location of the source is not revealed, only that its presence is somewhere within range. Whenever the creature with scent comes within 5 feet of the source, the creature pinpoints the source’s location.
A creature with the Scent sense can attempt to follow a scent, making a Wisdom (Survival) check to find or follow the scent. Creatures tracking by scent ignore the effects of surface conditions and poor visibility.
The typical DC for a fresh trail is 10 (no matter what kind of surface holds the scent). This DC increases or decreases depending on how strong the quarry’s odor is, the number of creatures, and the age of the trail. For each hour that the trail is cold, the DC increases by 2.
Hiding Your Scent
Water, particularly running water, ruins a trail for air-breathing creatures. Water-breathing creatures that have the scent ability, however, can use it in the water easily.
False and/or powerful odors can easily mask other scents. The presence of such an odor completely spoils the ability to properly detect or identify creatures, and the base DC to track becomes 20 rather than 10.