In my, uneducated, opinion, I honestly believe the coin you found was re-purposed for something mundane and the purpose/reason lost to time. Such as a washer, etc.
As for challenge coins they are actually used by just about every military MOS/AFSC/Rate etc, they are not limited to Special Forces/Special Operations. Although one story shows Army Special Forces as one of the origins of the original coins.
We call them RMO Round Metallic Objects, if you get Coin Checked at the bar and don't have it you buy a round for every one. If everyone has it the person who called for the coin check buys a round for everyone.
I have several challenge coins that have special meaning to me. Example.
SEAL Team 3 coin, given to my by SWCPO H******n. He had just transferred from Team 3 to DEVGU right before 9-11, so he didn't have a DEVGRU coin yet. While deployed I provide survival vests and training to the SEALS.
Green Beret coin, I swapped unit coins with a SSgt Vance, shortly after we exchanged coins he was killed in action.
75th Rangers Regiment Commander's coin, presented to members of our unit the 711th SOS for in country fund raising we did for the families of the two Rangers killed on hit night 18/19 Oct 2001.
Plus a myriad of coins from various units I was in or supported, deployment coins, and a few coins presented by Multi-Star Generals.
A link to some suggested birth origins of Challenge Coins, there are more theories if you dig deeper.