When I’m filling my bike tires, I put the cap on the ground next to the tire. In my mind, placing it up high, on something that can move, is just waiting for it to fall and bounce into an alternate universe.
I was going to suggest you look in your back-up suitcase.
In desperation I have formulated a more cunning plan. I put them in that strange little pocket in my jeans above the regular pocket. That’s what those are for, right? I expect there are a handful in there now.
Then where do you keep your pocket watch?
One fall my wedding ring fell off while I was cleaning up the garden. The following spring when I was planting new stuff it suddenly turned up. I had lost a fair bit of weight and used the experience as an excuse to have it resized.
In my defense, I did have the two chargers in separate bags, as well as my underwear and toiletries split into both bags.
For many years I’ve had this beautiful waiter-style folding corkscrew made of brushed stainless steel. It’s opened so many fine wine bottles over the years that it seems like an old friend.
I haven’t seen one like it for sale anywhere for many years, so I was most perturbed when one day it suddenly disappeared. Just … gone. I looked in all the usual places, then in less than usual places, then became obsessed and looked everywhere I could think of. For hours.
I did eventually find it. It was under the fridge. To this day I haven’t a clue how it could possibly have got there. I blame poltergeists haunting the previous house. In the current house, which is not haunted, it has always remained right on the countertop where I keep it.
Once this and other disappearing items have been investigated thoroughly, we will have solved matter transmission.
Anyone volunteer to write up a research grant proposal for that?
I can’t find my pencil. It was right here a minute ago.
Annnnnnnnnnnnd… scene!
I’ve been home from work for a few hours, cooked dinner etc… and just reached up and found a pencil tucked behind my ear (well, that’s the expression, but it’s more on top than behind). I was using a pencil at work… but could have sworrn it was yellow, and this one’s red. @Robot_Arm was yours red? If so, that’s very exciting.
If my experience is any indication, go to the nearest vehicle. It is underneath it, smack dab in the middle.
mmm
I once was looking for a pencil that I had in my hand while reading a book. Could not find it. I then held the pencil in my mouth while I lifted the book, to see if it was underneath. It was not. I took the pencil again and went on searching. A couple of minutes later I felt very stupid.
DAMN YOU GUYS!
I was reading this thread and started thinking about the time I lost my wedding ring, and when I glanced down at my hand MY RING WAS GONE! I have no idea where it could be. I woke up about 90 minutes ago, and since then I went outside to move my car and talk to the guys who are now replacing our roof. It might have come off in bed, but I looked and didn’t find it there. I’ll look around the house some more, but if it came off outside, I’ll probably never find it.
DAMN YOU GUYS! IT’S ALL YOUR FAULT!
(The only silver lining is that when I lost it ten years ago, we bought a replacement, before finding it [see here], so I still have that one…somewhere…I think…)
This thread is hilarious (though I hope the wedding ring reappears).
I suspect the tyre cap in the OP somehow rolled off the shelf and down the drain.
My contribution: a few years ago we somehow lost a kitchen knife (about a five-inch blade, so not a small knife). Our best guess is it somehow fell in the trash (perhaps while scraping a plate), but damn, that is a big (and potentially dangerous!) thing to go missing.
I was living in a four room apartment not far from the first full time job I got after getting my Masters Degree and somehow, even though it never left my apartment, my checkbook disappeared. When I went to write a check and couldn’t locate it, my first reaction was annoyance but, after a long and intensive search, that was replaced by utter disbelief. It NEVER left my residence, EVER, so how could it suddenly just not be there?!
I never did find it, and I guess I’ll never know what happened to it. When I finally moved, I thought it might show up after everything was moved out of the apartment, but no dice.
This is why I switched to Presta valves. 
I have long suffered from a similar problem. Seems like many of my hobbies involve disassembling things and managing the resulting array of small parts. I am 100% capable of removing a screw from an object, briefly setting the screwdriver down, then 1 minute later being completely unable to find the screwdriver I was holding moments before.
I have two theories.
Simplest and most elegant has to do what I think is called the coefficient of restitution. You pump up a basketball and test the pressure by dropping it from chest level to see that it bounces back up to waist level. So say it bounces back 60% of the distance it was dropped from. Each bounce is a little less energetic until the basketball is at rest on the ground. Well, I think that critical small parts that I have been focusing on and handling somehow develop a coefficient of restitution greater than one. That is to say each bounce is higher than the last. In fact, the more critical the object I am handling is, the more exponential the rebounds are. A one of a kind fitting from a vintage camera can bounce three times and obtain a velocity a significant fraction of the speed of light. An object like that is well on its way out of the solar system before I’ve even gotten down to look under my workbench.
My second theory is that the Twilight Zone episode, A Matter of Minutes, is actually a true story. In that show, our protagonists learn that the world is actually a series of spaces we only occupy for a minute each before moving to the next. The world is constantly under construction.If you were to go to one of those minutes scheduled to go live a year from now, you’d find mostly empty void. If you instead go to a minute planned to be live in about an hour, things are all constructed except final details. The construction supervisor explains that every once in a while, the building crew forgets to place an object. That screwdriver I was just holding literally doesn’t exist for a brief while. Eventually, when it returns to be exactly where I knew I left it, it was because an audit revealed the mistake and the lost object was replaced. Also possible that these construction guys are fucking with me. See also how two power cords can tie themselves in a knot in the 30 seconds I’m not looking at them.
In all seriousness (which goes against everything I stand for), I put a small bowl in the garage that I religiously use to hold valve stem caps while I’m airing up the tires on anything in the fleet.
And for the exact reason behind the OP.
It’s a light colored bowl that seems to work well for plastic or metal caps – Presta or Schrader.
I wanted to use one of those mechanic’s magnetic metal bowls, but … after some thought … decided against it 
On the plus side, I recently finished a 2000 piece jigsaw puzzle (Van Gogh’s “Starry Night”) that took me nearly a year to put together, and there were no pieces missing in the end! So, yay me.
Do you have crows/ravens/magpies where you live? One could swoop in and snatch it and you might not hear it.