Unmarked grave
The phrase unmarked grave has metaphorical meaning in the context of cultures that mark burial sites.
As a figure of speech, a common meaning of the term "unmarked grave" is consignment to oblivion, i.e., an ignominious end. A grave monument (or headstone) is a sign of respect and fondness, erected with the intention of commemorating and remembering a person. Historically, unmarked graves in an unintentional sense are usually done for financial reasons by families that are too poor to afford a headstone, though in more recent times this has been avoided partially because the standard of living in most countries have since improved and even in cases where the families still can't afford a headstone, one is usually donated from a family friend or other charitable organization. Apple, Inc. founder Steve Jobs is a recent example of a notable grave without a headstone.
Even when a person's remains are lost, a cenotaph may be erected. This is what happened to comedian John Belushi. The gravestone at his grave in a Martha's Vineyard cemetery was removed and relocated, after operators of the cemetery found many signs of vandalism and rowdiness, where his body lies. A cenotaph gravestone was erected at a nearby empty grave, to deter disrespectful visitors, leaving his actual final resting place without a marker. Another John Belushi cenotaph gravestone was erected by his family, in a Chicago area cemetery at the Belushi family plot, where his parents are now buried. Similarly, when H.P. Lovecraft's headstone in Providence, Rhode Island was stolen, a replacement marker was erected in a different location.
Criminals
Conversely, a deliberately unmarked grave signifies disdain and contempt. The underlying intention of some unmarked graves may be that the person buried is not worthy of commemoration, and should therefore be completely ignored and forgotten, e.g., Heinrich Himmler, Jimmy Savile.
Unmarked graves have long been used to bury executed criminals as an added degree of disgrace. Similarly, many 18th and 19th century prisons and mental asylums historically used numbered (but otherwise featureless) markers in their inmate cemeteries, which allowed for record-keeping and visitations while also minimizing the shame associated with having one's family name on permanent display in such a disreputable context. Plot E at Oise-Aisne American Cemetery (consisting entirely of soldiers executed for rape and/or murder) is a rare example of this policy persisting into the 20th century. More recently, the practice has been to cremate and secretly scatter the ashes of notorious criminals in some anonymous place. This was the fate of Nazi war criminals such as Hermann Göring, Fritz Sauckel, Julius Streicher and Adolf Eichmann. The remains of British serial killers Myra Hindley and Dr Harold Shipman were treated in the same way. Cremation and secret scattering of the ashes has the additional effect of removing all possibility of there being a grave to visit in the future.
Judaism
Within Judaism, in which contact with a corpse confers uncleanness (see Numbers 19:11-22 and Tractate Oholoth in the Mishna), an unmarked grave opens up the possibility that a pious Jew could become defiled without being aware that it happened. The Jews of early times, therefore, sought to avoid unmarked graves by two means: clearly designating cemeteries beyond the limits of their villages and cities, and making graves and tombs obvious by whitewashing them. This is the background for Jesus' comparison of the Pharisees of his time to white-washed tombs (see Matthew 23:27-28) and to "unmarked graves, which men walk over without knowing it" (Luke 11:44). Jesus warned that the Pharisees were defiling others by their hypocrisy, misplaced priorities, and selfish ambition.