They're physically surrounded by a far more advanced civilisation and it's literally invisible to them because it's incomprehensible to the ant mind, and it doesn't communicate by leaving scent trails.
We could be living inside the alien equivalent and we'd have no idea either, because we wouldn't be able to see it - never mind understand it. Maybe once every few million years something random and inexplicable would happen, but that would be all the evidence we'd get.