Scarlett has done that when she saw her own reflection in a piece of glass. I was honestly a little freaked out until I figured out what she was looking at.
When she did it one time, I literally looked between her ears and down her nose and sure enough, a reflection. I burst out laughing!!
Now we know to "sight in" whatever she may be looking at to understand what she is growling or barking at.
Reflections really get her hackles up. She obviously does NOT like other GSD's in her territory.
BUT, she was getting very agitated on our deck in February. Clearly, she was tracking something under the deck. We put out a trap and caught the intruder. That is one HUGE squirrel that will not be chewing his way under my house ever again.
At a point you need to trust their instincts and sense of hearing and smell. Past that point, you have to assume that it's something goofy that has alerted them. Since you are alpha, it falls upon you to sort it out.