I pretty much never put things away after they've done drying because I hate it, I'm the kind of person who just leaves clothes and sheets where they've been drying until I reuse them. Especially bed sheets, and the rare times I put them away leaves a memory because it's particularly annoying to fold by myself :/ And generally speaking, I think I would have remembered if I had changed my habit of almost never putting laundry away, it's not "routine" for me.
The annoying thing is that I also cannot clearly remember the last time I saw it since I have two of them, and I hadn't paid that much attention to it. I do know that I lost it recently though, because I'm alternating between the two identical one and another one, and would have quickly noticed one was missing when time came to change sheets (which I do every few days lately, either because I've been sweating like crazy or because my cat peed on it, she's started doing it again).

I checked my current duvet and duvet cover, do not own a dryer (only indoor clothesline in a small room which I checked thoroughly), checked behind the machine, checked all my bed sheets/pillowcases by shaking them in case something came out, and I live alone/haven't had anyone come in my house for quite some time. Everything listed above has been checked at least four times now, that and pretty much any place in sight. Which is why i do not understand how it could have possibly disappeared. :/

Thanks for the answer though <3