This looks like a crystal deposition disease in the extrinsic radiotriquetral ligament,
There looks like a subtle echogenic blush superficial to the bone, within the radiotriquetral ligament.
There is no hyaline cartilage on the dorsal aspect of the triquetrum as it is not an articulating surface.
It is however an enthesis for the radiotriquetral ligament and also the flexor retinaculum.
I think there are 4 options.
It could be gout,
It could be hydroxyapatite
It could be CPPD
It could be dystrophic calcification / scar tissue from an injury to the radiotriquetral ligament which would usually occur following a FOOSH (fell on outstretched hand) injury.
If you do a perfect lateral radiograph you will see a density if it is CPPD or hydroxyapatite.
It will be invisible on the xray if it is gout and if it is post injury scar tissue it will be invisible unless it has a significant dystrophic calcification component.
I think these are your options.
Steve