On sol 526 (January 28, 2014) Curiosity used its MAHLI camera to investigate one of the two wind-sensor booms sprouting out of the rover mast, part of the Rover Environmental Monitoring Station (REMS) instrument suite. The wind sensors on Boom 1 have not worked properly since landing, although they checked out fine during cruise.
Here is a second view from a different perspective, taken on the same sol.
NASA / JPL / MSSS
Curiosity REMS sensor boom 1 (from back)
Here are zoomed, contrast-enhanced looks at just the sensor parts of the boom from the two photos.
Detail view of REMS sensor boom 1, sol 526, front view
Detail view of REMS sensor boom 1, sol 526, rear view