Taught my dog to smell a clothing item, and she can distinguish the right person and follow the footfalls that person walked. She is pretty reliable up to about 18-36 hours old depending on weather conditions.
Just about any dog can do it, as long as you can find a good reward system for the dog. Mine loves playing fetch, so once we find the person we're looking for, that person gives her lots of attention and fetch play.
It took a lot of work, and a lot of help from other people, but she's really good at it now.