Hi Ray,
Great images from you as usual !
This location is interesting.
In this location deep to anconeus you are showing nicely the annular ligament / LUCL attachment onto the supinator crest of the ulna.
So there is certainly elbow joint capsule present in this location and hence SMI signal there may indicate some joint capsule synovitis.
There is however also a normal vessel that runs through this plane (recurrent interosseous artery) and some of your signal looks like it is coming from that normal artery which would be a normal finding. The artery always sits in that location on the surface of the joint capsule, just deep to anconeus.
Steve.