I had an ancient dog who spent most of her later years impersonating a carpet. We truly believed she was stone deaf and mostly blind, but then someone who was working on our house tried to walk up the stairs to the attic room where I was sleeping (just a mistake, he wasn’t up to anything shady) and she just exploded barking at him, straight out of an apparent coma. It was quite sweet to realize she had actually been keeping an eye on us all that time and knew.
We had an old German shepherd like that. Spent most of his time sitting like a bump on a lump. I was out late one night and tried to sneak into the house. Turns out Elvis was on security duty. In the dim light seeing 100lbs of glowing eyes, snarling teeth, and fur coming at me I about shit my pants. Luckily he stopped dead in his tracks as soon as he caught my scent and then went right back to snoozing.