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Granted, I'm no science person, but would dogs really be able to track off of a (former) puddle of drool in the desert? As others have pointed out, fluids tend to evaporate pretty fast on fry-an-egg pavement at high noon. Whether they leave behind much of a scent, I cannot say.
Urine doubtless would, based upon my experience on the streets of NYC, but it doesn't smell instantly; it takes a while baking in the heat for that signature to evaporate.
I should think "blood" is being concentrated upon by those who are concentrating upon it above all other bodily fluids, because in addition to leaving behind the expected biological spoor, blood would leave behind a stain; and in the pantheon of things likely to happen I should think that someone who hit-and-ran in the desert, then pressure-washing (or similar) the pavement to remove the bloodstains before they move on with their hit-and-run victim, is the least likely.
I really don't think you'd be able to see blood easily on the pavement out there. Whether a hit and run left a lot of blood would be another variable, many of them don't, the person ragdolls through the air and dies on impact with the pavement. Some blood would be there, but not a big puddle of it.
I don't think sniffer dogs sniff for blood, though, they sniff for the person's scent. Cadaver dogs sniff for the products of death. There may be specialized dogs that sniff urine, but I doubt it (and there would probably be quite a bit of urine scent out there, from other travelers, from wildlife, etc, not sure how specific urine smell is to an individual).