The still are unclaimed pieces of land. The Bir Tawil Triangle is probably the most well-known example. It is a piece of desert on the border of Egypt and Sudan. Neither Egypt nor Sudan claims Bir Tawil since they both claim a larger and more valuable piece of land (the Hala'ib Triangle) closer to the Red Sea. Egypt and Sudan are afraid that if they claim Bir Tawil, their claim on Hala'ib becomes weaker (it has to do with conflicting borders and administrative regions during the period of British colonialism; neither side ever controlled both triangles, so taking the small one automatically means the other side gets the large one).

There are other (smaller) pieces of land in similar situations, where not claiming a piece on the border is a strategy to be able to claim another piece, for whatever political or historical reason. For instance the states in former Yugoslavia have one or two such areas I believe.

Franz Josef Land, an uninhabited archipelago in the Russian Arctic, was discovered by an Austro-Hungarian expedition in 1873, but never claimed until the Soviet Union formally annexed it in 1926.