I'm not entirely sure what I was doing other than exiting power armor, but I just called Dogmeat. That is, I fat-fingered the keys and heard the voice actor yell "Dogmeat!". Then, Dogmeat came running. I've been lamenting the absence of just such a key, but apparently there is one. Yet, I can't figure out what it is. Does anyone know?

He may have just been under your cross hairs when you pressed E. You can call to your companions from a distance if you have your crosshairs/cursor on them