Memory is weird - we think that we are playing back experiences as they happened but actually we sort of construct them later. A bit like how, when you play a computer game which has characters and objects etc, if you could "see" what the computer sees, you'd just see a load of code like "Person JIFFIH interacts in FJGIDFG way with objexct DIDJODJ"
The graphics card knows what textures and 3-D shapes belong with each of those three codes and it goes and grabs them and puts it all together to create the image you see on your screen.
If you install something like a texture pack, or if the game files get messed up and swapped around, you will see a different person to someone else who has the same game installed.
Memory is a bit like that - we encode it like the computer code, and when we recall it, it's like our internal "graphics card" is finding out which images, sounds, smells etc map onto that code.
Also, there is apparently a thing where multiple people will occupy the same "role" in your mind and will be mentally swapped out easily. That is how when you have more than 2 children/pets you find yourself always calling them by the wrong name, or in a dream, when trying to recall it you realise that your sister morphed into your best friend part-way through the dream but you kept interacting with her as though she was the same person.
But actually, one thing which did come to mind @freakedoutseeingthings is can you go to a large supermarket or hardware store, and buy a carbon monoxide monitor? It's not that uncommon for holiday lets to have poorly maintained gas boilers since there are fewer regulations around holiday lets/in different countries, and because people only stay there for a week or so at a time, it's not picked up as an ongoing symptom thing. But carbon monoxide exposure can cause hallucinations, along with confusion, headaches, nausea and fatigue. I feel like there is a story in the news every year about some poor family who go on holiday and get killed by a faulty boiler in their holiday let
(And that has just reminded me that I got rid of ours because we only have electric heating, but I should get one to take on holidays!)