The turkey vulture has an unusually large olfactory bulb for a bird, and behavioural tests going back decades show it locating concealed carrion by scent. The key cue is ethyl mercaptan and related sulphur compounds produced in the first stages of decomposition.
The practical consequence is well documented: because odourants added to natural gas include mercaptans, gas companies have reported using circling turkey vultures to pinpoint leaks along buried pipelines. Black vultures, by contrast, have weaker olfaction and are often observed following turkey vultures to carcasses before displacing them.
The smell sense also has limits that popular accounts skip. Turkey vultures prefer relatively fresh carrion; strongly putrid carcasses are often avoided rather than sought. The old claim that vultures circle over dying animals waiting for death is largely wrong, since circling usually reflects thermal soaring rather than any target below.