It seems like we have a black hole in our house. Several categories in particular have a high rate of disappearance.
Hannah’s socks – I’m constantly buying more because I can never find any. I can ransack the house and find ONE of each pair but rarely a matching set. Most days, Hannah is not wearing matching socks. Then eventually they’re gone altogether. They’re not disappearing in the wash because I have a washable zipper bag that I put them in. Where are they going?
Cat toys – Of course she bats them around the house and they slide under furniture and in weird places. But, in our old apartment, we noticed two particular hedgehog toys missing, and we always said, “We’ll find them when we move.” Then we moved, and we never found them.
Plastic-ware lids – It’s just the lids disappearing, not the containers. Somehow I ended up with six of a particular size and only two lids that fit. This doesn’t work well for me.
My sister is coming soon to help me organize and de-clutter. Maybe some of the renegade items will turn up as we go through stuff. I want to track them down, because semi-useful things make me feel wasteful. Despite my best search efforts, I keep buying more socks even though there must be perfectly wearable socks somewhere in my house. And I don’t want to buy new plastic-ware just because 70% of the lids have disappeared. But they lose a lot of their usefulness when there’s no lid. “Here, Brett, I packed you some spaghetti for lunch, but be sure to hold it upright and cover the top with your hand.”
Maybe I have Borrowers! But that’s a strange assortment of items for them to be taking.
grit & grace 
Hehe, items will be found! No mercy!
H’s socks- the car. Otherwise, all the cool kids are wearing unmatching socks! Besides, you could start training H that whenever she sees a sock, she puts it in a bin in her room. Child labor? Maybe, but it’d sure be a fun party trick! Potty training might be more important that this however.
Cat toys- The cat ate them.
Lids- Have you checked the space between your dishwasher & fridge? I currently have one “lost” in a similar crevice in my kitchen :)
I recently did a purge of all of the singleton socks in the sock basket I keep on top of the dryer. It was astonishing. Where do they come from and where do their companions go? The weird thing was that there were individuals of socks that I never recalled seeing/owning pairs of…