As background on trailing the scent... I found this interesting...
"Scenting ability[edit]The Bloodhound's physical characteristics account for its ability to follow a scent trail left several days in the past. Under optimal conditions, a Bloodhound can detect as few as one or two cells. The Bloodhound's nasal chambers (where scents are identified) are larger than those of most other breeds. The number of olfactory receptor cells are 4 billion in a bloodhound, compared to just 5 million in a human and 200 million in a cat and 100 million in a rabbit[43] The surface area of bloodhound olfactory epithelium is 59 sq.in. compared to human's 1.55 sq.in. (10 sq.cm.)[43] The large, long pendent ears serve to prevent wind from scattering nearby skin cells while the dog's nose is on the ground; the folds of wrinkled flesh under the lips and neck—called the shawl—serve to catch stray scent particles in the air or on a nearby branch as the bloodhound is scenting, reinforcing the scent in the dog's memory and nose.[44] However, not all agree that the long ears and loose skin are functional, some regarding them as a handicap.[29]
Human Trailing[edit]There are many accounts of bloodhounds successfully following trails many hours, and even several days, old,[29][30] the record being of a family found dead in Oregon, in 1954, over 330 hours after they had gone missing.[14] The bloodhound is generally used to follow the individual scent of a fugitive or lost person, taking the scent from a ‘scent article’ – something the quarry is known to have touched, which could be an item of clothing, a car seat, an identified footprint etc.[29][45] Many bloodhounds will follow the drift of scent a good distance away from the actual footsteps of the quarry, which can enable them to cut corners and reach the end of the trail more quickly. In America, sticking close to the footsteps is called ‘tracking’, while the freer method is known as ‘trailing’ (in the UK, ‘hunting’
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, and is held to reflect the bloodhound’s concentration on the individual human scent, rather than that of, say, vegetation crushed by the feet of the quarry.[45][46] Having lost a scent, a good bloodhound will stubbornly cast about for long periods, if necessary, in order to recover it.[13][22][29] The bloodhound is handled on a tracking harness, which has a metal ring above the shoulders, to which a leash is attached, so that the hound’s neck is not jerked up when the leash becomes taut, as it would with a collar. The leash is at least long enough to allow the hound to cross freely in front of the handler, some handlers preferring quite a short leash, giving better communication with the hound, others liking something longer, maybe twenty or thirty feet."