I have one of these for my keys.
It keeps them together, but individual rings with their key(s) can be removed - the rounded end of the fitting runs round a narrower track on the holder, with a spring loaded opening for removal.

Normally, I only ever remove my car key on a regular basis, as I don't want the other keys banging round my knee when driving.
There is always a slim chance that another key ring could come off at the same time as I take the car key off, so I check that all keys are there, usually, when I take off/put back the car key.

Yesterday afternoon as I was about to go off in the car, I noticed that one key had come off and was missing (first and only time this had happened in several years). Fortunately, it was the least important key - a padlock to my bike shed, and I had a reserve key; I would just replace the padlock and have 2 keys again. I was more annoyed that I had lost the small ring with the special fitting.

The only logical explanation was that it had obviously come off when I had taken my car key off - 4 times in the previous 24hrs.; 2 of which were at home, and the 3rd was at my m-i-l's, and I'm sure I would have heard it click on the hard ground. I checked at home - around and in the car, in and chairs and sofas - in case it had come loose in my pocket and later slipped out -- nothing.

It was clear, it had probably come off at the 4th time I'd removed my car key, in a dirt/stone car park at a recreational spot about 30 miles away, we'd visited the previous day, up in the voralpen/prealpes (called Jaunpass).

I was in this car park, on the far side, approx. positioned where the dark car is with the boot open (top right of pic.):



The place isn't really a village, but is a small ski area in winter, and a nice walking area throughout the year, so it does attract recreational visitors. It also has a permanent campsite - more like an area of big garden sheds, as opposed to anything resembling a tent or caravan - and a couple or so bars/restaurants.



The car park is on the left, upper middle.

What was the chance of the key being there, over 24hrs. later, even if had fallen off there in the first place (it could have easily come off the previous day, with no chance of knowing where).

This is where logical assumption came into play.
- firstly, there was a strong chance it had fallen here - stony ground meant no noise when it fell;
- secondly, I couldn't imagine that the car park had been used much on a Monday - normally only busy at w/e's;
- and thirdly, anybody using the car park on a Monday could well be of a mature age, and not necessarily looking about on the ground - and they probably would have parked closer to the road and restaurant, or the start of the walking area.

When my wife returned home from work, I got out my bike, put on my leathers and off I went, with her words echoing behind me: ".... well if nothing else, your bike needs a run, anyway".
[It's one of my my (and many other FR/BE bikers) preferred runs, because of the nature of the twisty-turny bits on both sides of the pass]

I arrived 45mins later, got off the bike and walked over to where I thought I'd parked the previous day. More or less straight away, after less than a minute looking around on the ground, I saw something glistening in the evening sun, so I walked over, bent down, picked up my key and returned to the chain holder - not feeling at all surprised, and only just a little bit lucky.

After another fun 45mins ride, I got home, and to the amazement of both my wife and eldest daughter (who both admitted they thought I was wasting my time - 2 hopes of finding it: no hope and Bob Hope), I showed them the key.
For me, I'd have actually felt put out and cheated if I hadn't have found it.

So ..... a nice evening ride on a favourite stretch of mountain road; a recovered key; and a reassertion of the power of positive thinking.

Thanks for reading (if you've made it this far).