Rats are intelligent. They're very prolific. They can, and will enter through holes so small that you'll be amazed. You may have caught a few, but the others are now staying clear of your traps. I've shot them, I've set traps for them, I've poisoned them and all I did was to inconvenience them. Be cautious with poisons. The real joy is when they die in a wall. The odor is exquisite and persists for a year or two. Finding it in a wall is not easy. If you are unlucky and poison something larger in a wall, like a possum, you may as well move out.
Someone is feeding your new friends. I finally found the source of my problem which was a neighbor, several houses away, that fed her dogs outside and left food outdoors for them.
There was an excellent piece on these charming rodents that covered the entire front page of the Dallas Morning News House & Garden section on Sunday, 11/20/1998 titled "Oh, Rats." Seems Dallas has been taken over by rats. It has. They are an epidemic there. Maybe you're seeing them as they migrate south to Austin.
Move north, at -20F I no longer have rats.
Good luck.
Boden