Tracking is a useful skill that PCs often like to have. It's great in wilderness regions where there are more or less obvious tracks. Skilled characters can read tiny marks and track target animals or people even in terrain where most people would have trouble spotting anything unusual. There are even ways and rules for tracking in urban settings such as bustling cities.

Chances of success can be essentially 100% - even a person with no skill can follow fresh footprints through clean snow or thick mud - right down to basically zero. Following a person through concrete city blocks an hour after they've passed is nigh on impossible without magic or science fantasy technology. And there is the full range in between. Games often use modifiers to a character's base tracking skill based on terrain, weather conditions, and time elapsed since the pursued target passed by. The good thing is these modifiers can be stacked and easily take the chances of success over 100% or below 0% - corresponding to the situations described above, for example.

In some cases the chances of successfully finding tracks can be so negative that the character can have a far better chance of finding the target by just choosing a direction at random and walking.

Transcript

Han: Echo Base, I'm at Luke's last reported location.
Han: I search for tracks. 6.
GM: In a blizzard? At night?
Han: Yeah. Do I see any?
GM: You can't see your own tracks.
Han: Yeah, but can I see Luke's?
GM: As it happens, nope.
Han: Oh wait, silly me. Obviously I'm not going to see Luke's tracks.
Han: Can I see his won-ton's tracks?
GM: You can't even see the ground!
Han: Hmmm. I better take off my goggles.