Many of the products of decomposition are persistent and long lived. In particular, adipocere (a waxy fat) can take many years to break down. It probably traps other volatile decomposition products, as well, releasing them slowly.
In addition, insoluble minerals from the body (including phosphorus) can remain in the soil, providing chemical markers and changing soil chemistry. This combination can change the vegetation on the grave from the area norm (although soil disturbance during grave digging can do this, too). It can be pretty significant.
There is plenty of ongoing research into this topic, and there are developments of instrumentation to provide more reliable detectors then dogs (which are enthusiastic and pretty good, but are expensive to train and maintain).
There are some notes here on dogs working a site over 150 years old. They did not find the burial site (which was post-decomposition), but seemed to detect the primary above ground decomposition sites. By this standard, 23 years is pretty recent.