I find single keys all the time, sometimes sets.
Found this set yesterday.
A few inches deep, they have been there awhile, a few years, anyway.
This one has a Hen House, (local grocery chain), discount card attached and it says postage paid, so I can drop it in any mailbox, but I hunt in the area of this store sometimes so I will drop them off next time I am in the neighborhood.
The store can track down the owner and return them...bet they will be surprised when they get that call.
I found 2 other great things attached to keys in the past, another reason I dig all zinc signals I come across.
One was a silver Tiffany key ring and a whole set of keys...
Another was this lonley key I found buried in a park, another zinc signal.
When I dug it I saw there was a little string attached to the key, but when I looked closer this was no string...it was a very thin 14k gold chain.
Dig those keys.
Not only are there some pretty cool keys lost out there but you might even get a surprise from time to time.