I used to live near the Louisville Swamp nature reserve, southeast of Minneapolis, MN, along the Minnesota River.
Many turkey vultures cruise the area. A few years ago I (and my wife) saw a foraging kettle of them (they sometimes stack in levels, as many as three distinct altitudes, and follow each other) attracted to an isolated fallen tree's trunk in a sort of prairie/ oak savannah - a couple actually landed on the horizontal trunk, hopping around and fluttering away and focused on a small area. The rest lowered and circled, going away and coming back, for a long time - we watched for at least fifteen minutes.
When I went to see what the focus was, I hit a stench of dead animal but no carcass. It traced to a (badger sized) hole in the ground under the tree trunk and angling under a large boulder (glacial erratic, common in that park) and from the smell something was dead in that hole and had been for at least a day or so - but more than six feet under and not even slightly visible to me, looking into the hole from close range. No unusual number of flies or other insect activity at the surface, no fur, no bloodl.
I can't think of any way the vultures would have focused on that spot except by detecting at fairly high (soaring) altitude, and tracking to its source, the smell.