I believe there is a difference but I'm not sure what. DH comes in the living room and says "Where did everybody go?" when nobody has been here all day except the two of us. Or he'll come in and say "Where did that guy go?" and point at the spot on the couch where I have been sitting recently. Or we went out to eat and he came back from the restroom and said "Where is the other lady?" I usually just say "I don't know" but sometimes he gets frustrated with that, and wants me to find them. So far he has never acted as if the phantom person was still there, it's always as if they were here earlier and now they are not. He doesn't try to talk to them or anything. This happens almost on a daily basis.
In simple terms, an hallucination has to do with your 5 senses. You may see something that isn't there (sight), hear something that no one else hears (hearing), you can even smell, taste & feel things that are not real. Delusions have to do with a belief that something has happened or someone did something - like someone stole your money, your wife is unfaithful, you sold property you never owned. My DH told people he'd been cheated out of a business deal and sounded so real he actually had people ready to call an attorney or accountant until I stopped them. Unpleasant things had happened in business, but not what he told people. He also accused me of getting a divorce, being unfaithful - like I had the energy! -- and none of it was true.
If DH saw someone in the house I'd tell him that I told them to go away, told them in front of him as if I, too, saw them. It usually worked. For being unfaithful-- he raged at me & when I said, 'but I'm right here,' he yelled, 'oh, no you're NOT.' When I denied it, he called me a liar, and I certainly could not admit it. It's untenable, but I usually said something like, "I'm sorry about that, honey, Let's have some ice cream."
Judy, it sounds like your DH is not always sure who you are, he may be thinking that his wife is much younger than you now look. You appear in this room or that room and he wonders where that person went. You can always say 'he went home' or whatever you think will not frustrate him. It's the nature of the disease.
He may have access to the memory that someone was sitting there and realizes now they are not. But he doesn't have access to his own memories of the sequence of events over time. That's a specific function in the brain seperate from storing or accessing specific memories.
Each moment is a still picture. Moments strung together in memory feel like movies but we do the editing. We might pull out a short sequence and give it more meaning; but, we know we are now, that was then, our thoughts about that memory are now about then - and so on. We understand point of view and that gives us understanding of the relationships of things in time.
Any and all of those functions can build up plaque or it might not be in the memory areas so much.
My wife gave lectures to the hall mirror and chatted with the interesting person in the bathroom mirror. Neither of those lasted but they did go on for some time. She interpreted the image as being listened to and her needing to say things. Some might see their image and not be able to deal with the concept of reflection - so they are threatened or unsure who that is.
If we have it in our reality that there is supposed to be someone sitting there even though it's you right there in another chair, that can be quite upsetting. What if you tried an approach like "she left...she said to say goodbye to you". I wonder if providing closure of the inconsistency would defuse the concern where an "I don't know" response which does try to deflect it - keeps it open as an issue.
My husband saw a woman and child. He said seeing them didn't upset him. He would just ask me if I saw them, too. I told him the truth because I didn't know at the time that he had dementia. Now, if he said it, I probably would just say, "they must have gone, since I don't see them now."
Judy, My Dh does exactly the same thing...thinks there are people sitting on the sofa, or the kids ( Now in their 40's) are in bed.. Today at the hospital, he said "where did the lady go who was sitting with us"...there was no lady. Isn't it amazing how they pretty much say and do the same things. It took me a while to just say they had gone home.