The other night I'd just got in from work and fed and walked the dog, who was fine, completely normal. I went into the kitchen to start cooking my dinner and the dog came BOLTING in, hid under the table, shaking. I went back with her into the living room where she stuck to my legs, staring at the door to the stairs, prickling and reluctant.
I opened the door. There was nothing there. So I went back into the kitchen, finished cooking, all the time with dog under my feet and absolutely adamant that there was something scary on the stairs. Sat down, started eating.... stairs door started to rattle.
It was the wind. THE WIND. My upstairs window was open and the wind was just coming in at the right angle to make it down the stairs and rattle the latch. My dog was afraid of the wind.
Point being, dogs can and do react to things that, to us, are perfectly normal and harmless. They are idiots.