Either there's an animal you can see that she's hearing or she's just getting old. My dog did that too. He would just go outside and stand there then come back inside.
Discussion
Probably the latter. Even though she's still pretty spry, she's probably around 13-15 yrs old. Shelter dog, so not sure. But Daisy will start barking incessantly after shes been outside, so then we cant sleep for the barking. Then back in the crate, and she'll calm down.