Hi
Zoomed in just a tad too close to show us where the cable goes at the hub end.
However, assuming there are no warnings showing, my guess would be brake lining wear indicator.
If you back off just a smidgeon, so we can see the caliper and hub carrier, I'll know for certain.
Of course, someone with specific SX knowledge will recognise it from the existing pic and post the answer while you're out taking another shot!