My folks were big into raising some odd strain of Alaskan malamutes. Our malamutes were enormous. Very friendly towards all people, even night time burglars, and fatally vicious toward all dogs. We had them in a one-acre high-fence pen behind the house, and every spring the female would den up. As the oldest kid, it was my job to go out and find the den, and see if the female would let me in to get the pups. If not, and it was always no, heck no, with lots of big bared teeth, growling, snapping, then dad would come out and help me. They hid those dens unbelievably well. You'd think an acre is pretty small and that you'd be able to find the den easily. Not my experience. They were well hidden, and like the one commenter above says about his local wolves, our malamutes would always pick a very slight rise under some healthy spruce, and use the root system as a roof.
For those who might not know, Malamutes are big, strong sled dogs originally from Alaska and probably not genetically too far removed from wild wolves. They look like wolves, act a lot like wolves when they get out of the house or pen, and howl like wolves. I used to sit in the living room and howl with the dogs. Or put on the now-old Robert Redford wolf record from 1972 or thereabouts. The dogs would hear those howls and immediately sit on their haunches and start howling like mad, ears all perked up. If me or dad got into it with them, they went nuts, pacing, looking out the windows, barking. They were wondering where their pack was. Anyhow, if malamutes are any indication, then finding a real wolf den in wilderness is going to be very difficult.