Give her time, 5 months is a bit young yet to expect much of a watch dog. Give her until about a year old before you expect her to sound the alarm. Since she is a puppy, she is naturally not going to be dominate and therefore not be protective of her territory. Biologically she doesnt think she owns any territory yet. She just needs time to mature.
I've had a couple of dobies, one would bark his head off at ANYTHING (and I do mean ANYTHING...drove us crazy), the other would bark if it suited him...never did find out exactly what suited him.
If you're looking for a watch dog, they have to be trained to be a watch dog. Although some breeds have a proclivity for it, they still have to be trained. They don't know what you want of them instinctively.