I have had several nests this year and have seen a lot of them hovering around the ground, hunting, just like you. I have eliminated 3 nests and still see them. If you have an area with leaf litter, check extra closely. In previous years I used to find the nest in a part of my back yard that gets a lot of afternoon sun, but not much morning sun in a fairly open, grassy, dry, area. This year they were under bushes and in other areas with leaves piled up as mulch. It can be a little hard to see the coming and going at the nest entrance when it is hidden in the leaves.
Is it possible that the nest isn't on your property but maybe on a neighbor's land? I don't know how far they roam from the nest, but I have seen them down the street from me and wasn't sure whether it was from the nests on my property or not.
As for getting rid of them once you find the nest, a method I have never had fail: Wait until a while after complete dark and pour some honey down the hole. Within a day or two they will be gone. I have found this to work more reliably than using a can of wasp poison, and it has the advantage that it doesn't leave a residue you have to worry about poisoning your kids or pets.