Recent excavations at Leang Bulu Bettue, a limestone cave on the Indonesian island of Sulawesi, have yielded a collection of flaked chert and limestone artefacts with cortical surfaces that had been deliberately incised prior to or after the knapping process. The markings engraved on these artefacts, which were recovered from deposits ranging in age between approximately 30–14 thousand years ago (30–14 ka), comprise cross-hatched patterns and other non-figurative imagery. This behaviour is of interest because of the almost total absence of portable art in the Pleistocene record of Island Southeast Asia, and the long-standing idea that the early modern human lithic technology of this region was fundamentally simple and remained so over tens of millennia. Here, we take stock of these incised stone artefacts from methodological and theoretical perspectives. Our findings suggest that unless one is specifically examining cortex on stone artefacts for these fine incisions, they are easily overlooked, and hence, we focus on how to improve detection of these faint engravings. We also consider why the Leang Bulu Bettue inhabitants engraved stone tool cortex, a practice we regard as an enigmatic form of portable lithic art and an apparent example of the creative process being as important as the end product—if not more so. We conclude that otherwise unremarkable lithic assemblages in Island Southeast Asia and beyond may potentially harbour hidden evidence for symbolic content in the form of often barely perceptible markings on remnant cortical surfaces.