Now the mornings are dark when we get up, Red can see this ‘stranger’ through our patio doors to the conservatory . She initially just woofed at it. I have taken her up to it so she can see herself and me in the glass and I have opened up the door and let her through so she knows nothing is there. This morning she has done full on barking. So strange hearing this bark as she has been fairly silent up until now. She has barked more than Sky has in 10 years! She is now standing looking bewildered at the door as it gets lighter .
We're getting the same from our 6 month old. Whenever he barks, I go there and look at what he is looking at, and then give a reassuring word that there isn't anything there, and then walk away. It works maybe 80% of the time.
Lucky did this when he was about 1yr old - to his reflection in mirrors, shop windows, glass doors, and once even at the oven We just distracted him, and somehow he grew out of it - there's a mirror in the lift which we take every day, and he will look at me in the mirror, but he never looks directly at himself. I don't think he understands, but has just decided to accept it.
I'm having this with Cass at the moment, since I've moved her crate out of the kitchen and replaced with ordinary bed she can see out of the large window behind, the lower sill is probably only 15 inches or so off the floor. Once it's dark she barks at her reflection, to start with she was quite cross but now we just say, oh that cheeky black dog is here again, same time every night! She just wags her tail then and wanders off!
An interesting article on dogs self awareness https://thesciencedog.wordpress.com/2017/08/23/does-this-smell-funny-to-you/