Possibly looted over the years before archeology became big. Easier to sell a gem of ‘unknown provenance’ than a whole, very distinctive armband.

On the other hand, might’ve fallen out or the like. 2500 years is a lot of time for wear and tear, after all. Or been pryed out by the owner for some reason or another. Sadly, I don’t know the details for most of the artifacts I share on here - some have fascinating histories, others we can only speculate on!