Ants in Transit

There's an ant highway passing through my apartment. Little black ants enter from the light well through a crack on the side of my bedroom window. They cross the window sill, go down the wall and out the door, and follow the bottom of the wall through the living area to the kitchen, where they cross the room along the seam between the wood and tile floor, run along another short wall and go out the door. Outside, they climb up the edge of the door, across the wall and the top of my kitchen window, and through a window into the neighboring apartment.

The traffic goes both ways, and at any given time there must be a thousand ants in my apartment, but they're just passing through and don't seem to have any interest in what's going on here, so I leave them be. There was another route in my bathroom, coming out of a hole in the wall and heading to my wet sink where they'd drink. I tried to wash off their chemical path and even sprayed some insect repellant on the wall, but as soon as it dried, they found their exact same path again, so I gave up. But when I came back from traveling for a month, the water had dried up and they'd gone. Now I'm just left with the one ant highway and the question of where the heck they're all going.