She may also just be enjoying a wild run, and will come home when she's good and hungry. I once looked after a friend's male dog who loved a good overnight run, and was a mighty escape artist. He could disappear behind a single blueberry bush, despite being a sixty pound Shepherd. I recall one evening searching along the road, calling and calling, and I could see the great lout hiding in the edge of the woods, watching me. He once jumped through a shut glass small window on the second floor of a shed he'd been locked into (having previously chewed through the door of his kennel). He landed, a good ten foot drop, on his feet, took off, and was home none the worse for wear next morning.

Which is to say, take heart and keep looking. She'll be found.