Why bother with fiction when you can be horrified by everyday life? We've found a paper about how to locate bodies in "clandestine graves" by analysing the gas emissions they give off.
Odor Analysis of Decomposing Buried Human Remains is your must-read going into Halloween weekend. When we get murdered and buried in shallow graves, our bodies give off a variety of gases that eventually seep to the surface and can be sniffed out. (Fun Fact: Most "clandestine graves" are 60 to 75cm deep. Second Fun Fact: It takes an average of 17 days for the gases to reach the surface.) Traditionally, the sniffing was done by dogs, but dogs are trained to sniff for only relatively fresh corpses, and every time a law enforcement agency gets a new dog, the dog has to be re-trained.
In an effort to come up with a permanent tool to sniff out human remains, this study analyses the gases that buried bodies emit. Overall, there are 478 different chemicals, but most are given off by any decomposing body. Thirty of those chemicals, when taken together, make up a signature that's distinctly human. These gases can vary, depending on the soil in which the body is buried. The report states,
"These 30 compounds were produced at the corpse level (below body) at various concentrations and migrated upwards through the soil column unmodified by bacteria, potentially indicating that varying soil types and their resident microbial populations would not influence the presence, liberation, or detection of these compounds. One compound, carbon tetrachloride, slightly increased in abundance as it migrated upwards through the soil column, possibly indicating microbial transformation of a base compound into carbon tetrachloride."
If you want to go a little deeper, I suggest taking a look at the gases, picking a couple and finding out about them. I found out that toluene is a solvent regularly added to pain thinners and undecane is a sex attractant for the female olive fruit fly.