Early writing or sign systems were fragile. They could disappear leaving seemingly little trace in the systems that followed. This is true of the ancient Indus sign system. It is also true of the proto-Elamite one.
"Illegal excavations and looting of archaeological sites in parts of the Indo-Iranian borderlands and regions of South- Eastern Iran and Central Asia have been rampant over several decades. Archaeologists have attempted to minimise the damage caused by the plundering of sites by studying and publishing artefacts abandoned by looters on-site, or those recovered by security forces," write the authors.