I have a 6 year old greyhound who started to pee/poop upstairs when we are not home, even for a short time when he has been taken out. He goes from 7:30pm to 7:30am without an accident and the door to the bedroom is open. He never goes upstairs when we're home, even when coaxed. He weighs 80 lbs and I cannot physically carry him up so I can't reprimand him at the spot. What can I do
This sounds a lot like seperation anxiety. He doesn't deliberately misbehave to annoy you.
First things first:
If you want to reprimand a dog (or any animal), you have to do it within 3 seconds of the unwanted behavior. Dragging your dog to his poop upstairs and yelling at him is not going to change a single thing. Your dog doesn't understand why you are angry at the poop because he doesn't connect the reprimand with his earlier action of pooping there.
Now to his behavior. If he poops in the house even after he was outside (and had the chance to do his buisness) because you're away for a short time, he probably has seperation anxiety. Unfortunately, this is very hard to treat and I cannot give you a simple solution.
It's best to contact a professional dog trainer and ask for their help. By that I don't mean a puppy class, but someone who analyzes your dog's behavior at home and then tells you what to do.
Blocking the way upstairs like Eager to Play suggested is a good start, but might not have the desired effect. Providing a potty pad upstairs is an alternative, but it just deals with the symptom, not the cause.
Classical music or talk shows seem to reduce stress in dogs, as discussed in this question How effective is leaving a television or radio on to comfort dogs when away?
One possible solution that comes to my mind is to play or walk with your dog before you leave the house to make him tired. We have a list of different activities in this post. Hopefully, he will be somewhat exhausted or tired and use his alone-time to either relax or sleep. But even that is not guaranteed to work.
Your best chance is a professional dog trainer.
Since this is only happening when he is alone at home, it may be difficult to remove the behavior through direct training. My first recommendation is to prevent your dog from going upstairs when you are not home. Baby gates are often useful for this, or just strategically placed objects blocking him from going upstairs. See what happens when he is not able to go upstairs at all. Does he do his business in the rest of the house? Or is he successful at holding it? That will give you some more information about what the issue is.
If he does not have accidents in the rest of the house, then my recommendation is to keep him restricted from the upstairs for a few months and then try taking the gate away. See if the problem starts again. By that point, the habit may have been broken and it may not start again.
If he does have accidents in the downstairs area, then it means that he likely does need to be let out during the day and that he is using the upstairs area since that is less strongly associated with "don't pee or poop here." It is not unlikely for a dog to need to go out more during the day than at night since at night he is mostly sleeping and probably isn't eating or drinking much.
If this is the case, there are a few things you can try. If you feed in the morning, make sure that you are feeding early enough for your dog to go out at least half an hour after eating. If you are currently free-feeding at all (e.g., leaving dry food out all day for him to snack on), transitioning to set meal times may help. You should also talk about the issue with your vet to make sure there are no health concerns.