Caitlin seems to have an extremely comprehensive and detailed mental picture of all the places she normally goes.
If anything looks different to her, she starts barking (from a safe distance) at the suspicious object.
I used to put all my energy into getting her to shut up and come indoors, but now I go out and stand beside her, trying to follow her gaze to the source of the outburst.
Yesterday it was a bath towel that my grand-daughter had draped over a garden chair. Ashley had been playing with Caitlin in her paddling pool earlier in the day, and had left the towel there after drying her off. When Caitlin came out later and saw the unfamiliar shape on the chair, she went ballistic. She calmed down as soon as I picked the towel up, showed it to her and let her sniff it.
This morning as we came out of the house for our early walk, she started barking ferociously while staring across the street. Our neighbor had been clearing his bank of ivy and had left several bundles on the parkway. It took several minutes of coaxing to get her to approach and investigate the scary mounds.
Sometimes it's easy. Last week the barbecue had been knocked over by a visiting dog, and Caitlin didn't recognize it. I righted it, and voila! it was familiar once again.
Sometimes I can't figure it out. I'm not sensitive enough to appreciate the subtle change, and I suppose sometimes perhaps it isn't visual, maybe the scent of a raccoon or opossum that my poor human senses are too weak to detect.
Nevertheless, she seems to calm down once I have validated her concern by trying to understand, and usually she returns happily and quietly back into the house with me.
I wonder how much better a parent I might have been if I'd been patient enough and sensitive enough to do that more often with my step-sons.