I met a dog that had that exact look long time ago when I was still working for the phone company... the one I met was probably under a year old, so not quite as large as the one in the pic.
I was tracking down a bad spot on a line and needed to get into a phone pedestal that was located in the very back point of a pie shaped yard. To get into the pedestal, I had to wiggle down between it and the back point/corner of the yard. It was a tight fit but I needed to fix up the old cracked insulated wires and it was going to take some time.
Before I started working, I'd checked to see if anyone was home first, they weren't so I rattled the gate and called out to see if there was a dog in the backyard or not. No barking or other signs so I got to work. Just as I'd settled in to start rewiring the pedestal... the young male came out of the back garage door and growl slightly. I was in no position to do anything and knew any sudden movement would be a bad idea... so I just bluffed. I sternly asked him "what are you talking about"... that's when the look you see in the above pic happened, he sort of made a whiny sorta "huh" sound, while turning his head slightly and just relaxed, sat down and looked questioningly at me. After a moment went by ( and I felt relieved to say the least ), I said to him... "It's ok" and motioned him to come on over. He came over for a bit, let me pet him some and then moved away, then just laid down and watched me work. When I finished and got up, I gave him a little more petting and talked a little more with him ( it was a one way conversation other than him wagging his stubby tail a bit ).