One reason that I abandoned my beloved Blackberry for a less-loved Samsung Omnia is that it has a 5 megapixel camera which has encouraged me to document more regularly "archaeological" aspects of my everyday life.
Every day I walk leave Merrifield Hall on the University of North Dakota's campus past one of the oddest and yet most traditional pieces of campus graffiti. Unlike more urban campuses, UND's campus is remarkably graffiti free. Even restroom poets, so common to the semipublic space of university campuses, seems to be scarce. So this one example of campus graffiti really stood out. Someone had scratched a Chi-Rho on the inside doorframe at the southeast corner of Merrifield Hall. The Chi-Rho is an important symbol to Christians, representing the first two letters of Christ's name in Greek. According to most of the major sources for Constantine's reign, the Chi-Rho appeared associated with the battle of Milvian bridge after which Constantine showed a particular dedication to the Christian cause. It was famously incorporated into Constantine's military standard (called the labarum). By the later 4th and 5th centuries, the Chi-Rho became a symbol of the Christian cause appearing in both formal inscriptions, mosaic decoration, and graffiti.
This immediately reminded me of ancient examples of graffiti -- especially the Christian graffiti found at various points along the Hexamilion wall in Greece. While none of those graffiti were Chi-Rhos (that I can recall), several cross graffiti were located at doors entering towers or at gates into the fortress. (See: T. Gregory, The Hexamilion and the Fortress. Isthmia 5 (Princeton 1993), p. 126, ill. 23.). Scholars have often interpreted such graffiti as being apotropaic; that is designed to ward off evil of both the human and spiritual kind. Thresholds such as the door of a building or a city gate are liminal spaces (quite literally) and are unstable places being neither within the protected area of the building nor safely outside and away from protected space. The vulnerability of such places often prompted appeals to divine powers to protect the space.
Of course, in some instances, the evil or restless powers have already infiltrated the interior space and the apotropaic marker -- like the graffiti on the Hexamilion -- are reminders that the bad things of the world can't always be kept at bay.