Oh, I have seen them. I haven't hunted mule deer, but I have seen them. Again, that is why hunting in the rain can take that advantage away from the deer. Now, if you move through the woods like a bulldozer, a blind deaf and dumb mouse can hear you. What so many people forget is that the bottom of the hooves of deer are like those of cows, and horses, only smaller. The outer edge of the toe nail is hard, but the inside is soft. The deer use that soft portion to feel vibrations through the ground. That is how they often are first alerted to something moving around them, long before the head comes up, and those ears turn until they hear an audible( to them) sound in the distance. Hunters have to learn to walk softly. Most don't.
To enhance your senses, take your shoes off, and walk barefoot. After you get used to avoiding sticks, stones, and other things that can hurt your feet, try it wearing a blind fold, and follow a course that has a rope strung from one tree to another. By blinding you, you force your brain to use your other senses, and you will very shortly begin to hear so much better, smell and taste better, and, of course, feel so much more with your hands and feet. You will also learn to move slowly, and quietly in the woods, doing a Fox Walk, where you unlock both knees so you can feel the ground with your forward foot for sticks and stones, and stickers, while your weight remains on your back foot. When you find a spot that you can safely put that forward foot down, you will learn to put it down straight, and flat, so that anything that is under foot and will snap, crush, or break will produce very small sounds that are actually muffled by your own foot.
Compare that to shuffling through the forest turf, wearing both workboots, and overshoes, or wearing waders, and busting through brush instead of staying to worn game trails.
If you have a long way to go to get where you want to hunt, you are going to want to get there quickly, before daylight in most cases. Just stick to worn game trails, and take the time to trim off branches along the way that would otherwise strike you, or rub against you on your way into the stand in the dark. It can save you some nasty scratches. When you get within 200 yds( 600 feet!) of where you want to be, that is when you want to stop for awhile, wait for the forest noises to quiet- as most are alerting everything else that some freight train is coming through the living room( YOU!), and then quietly switch to the fox walk to cover those last 200 yds.
If it is raining, it really does hide the noise. Most deer- even mule deer, will stay in their beds until its daylight during a rain, because their ears are their primary defense system, as your eyes are yours. They wait for daylight before getting up and moving. They also seem conscious of changes in barometric pressure, as all animals are, and I have seen whitetails stay in their beds until midmorning when the rain stopped before getting up and finding a meal. If you see that kind of behavior during your scouting, (pre-hunt), you might was well stay until later on those rainy nights, before going to your stand, and not have to sit in the rain for 2 or 3 hours while the deer get a few more hours sleep, before they move.
All deer tend to bed down in localized places that are out of the wind. By enhancing your other senses, you can be more attuned to feeling those places where the wind is not so bad. With less wind, their ears work better. Of course, in a steady down pour, deer just have to wait it out, as the spattering of raindrops makes noise enough all around them either hitting leaves in trees, or hitting the ground that they either can't really hear much with their ears, or they can't hear much through their feet, until it gets really close.
Good hunting.
