Did you find the Fox?
Sound advice given in above replies.
Been back many times looking for Fox. Look hard.
I've taken a Beagle with me to do the trailing.
As far as the missed Fox...
I've gone back the same night and kill 'em or missed 'em again. And sometimes they don't respond to the second show.
Greys have a very small home range. So the missed Fox is still there, within earshot.
The missed Fox may hang up then sit, pace back and forth or bark, yip.
Or all the above.
Note where it hangs up. Next time working that spot, send you partner to the spot where the Fox hung up. Have your partner signal you when he is setup and ready. Tell him to be ready when he hears the caller.
You stay at your original calling spot and work the call. Use the same sound as last time. That should put the Fox at the end of partner's gun barrel. That's one method you won't read in a book.