There’s one peculiar case where an American founded a micronation somewhere in Africa, in territory disputed by two countries. What makes that one odd is that each country claims it’s the other’s territory. Apparently, it’s in the middle of nowhere, and a pain in the neck to maintain control over.
Huh. That sounds like one that could be successful. If both countries disclaim it, then it has a good start on a claim to sovereignty. The operative part of “it’s not mine, it’s yours,” is the “it’s not mine” part. The big question with all such claims though, is what happens when conflict arises. That is the test. I can declare my backyard to be Catistan, and then go about my life as usual, and neither my state nor the US will care. But if I declare it to be Catistan, stop paying taxes, start stockpiling weapons and annexing territory from my neighbors or manufacturing illegal (outside of Catistan) substances, Catistan will stop being a thing pretty quickly.
ETA: This is why “defensive indifference” may get you left alone for a while, but I don’t think it actually leads to sovereignty. You either need positive acceptance, or to win in conflict to show sovereignty.
It’s Bir Tawil but it is more complicated than that. Due to two different maps being drawn during the British Protectorate era there are two claims to Bir Tawil and the Halayib Triangle, one is Egypt’s and the other is the Sudan’s. The claims are in an exclusive or (XOR) relationship so that if you have one region you don’t have the other. Bir Tawil is a worthless piece of crap land while the Halayib Triangle is (relatively) fertile coastal land. Egypt claims the 1899 map giving them the Triangle and the Sudan Bir Tawil. Sudan claims the 1902 map giving them the Triangle and Egypt Bir Tawil thus if either country claims Bir Tawil then they are implicitly accepting the other country’s claim and will give them the valuable Halayib Triangle.