The group I work with has all kinds of materials donated to us to work with. We use human training materials in different stages of decomposition. It could be a piece of bloody gauze from oral surgery or a rug that once had a body wrapped in it.
A dog can detect human remains from 3 minutes after death. If someone was killed on a rug and only laid there for 5 minutes, my dog can sniff that rug out even after the body has been moved. So, even when it appears the body is long gone, the sludge from decomposition will remain mixed into the soil.
I fell asleep while responding last night. Sorry.
CAUTION..THIS EXPLANATION MAY GET A LITTLE GRAPHIC.
The oldest grave 1 of my boys have found was late 1800-1900, but they are now using dogs to locate burial grounds in digs that are much older.
The group I train with uses only human training material. Anything from a bloody gauze from oral surgery to rugs that once had a body wrapped in it, to bones from a mummy. If a decomposing body (7 minutes after death)has touched something, my boys can detect it. Even after a body is long gone, that scent is in the soil where it once laid.
Sick story. While training, Bull kept hitting on a pile of dog bones. He isn't trained to detect anything but human remains. I ended up calling the local police. Thankfully, that cop trusted my dog's ability and collected all the bones and sent them off. In that pile of bones was a finger from a human. About 5 years prior, there was a man that had died of a heroine overdose about 300 yards away. They didn't find him till 10 days later. This dog had run off with that guy's decomposing hand and they speculate that he too died of a heroine overdose.
My dogs can't, but a couple in my group are learning to locate bodies under water now.