Could this be a cosmic ray hit on my cameras sensor (CMOS, DSLR)?
It's a track about 50 pixels long, at 4.1 $\mu m$ per pixel that's 200 $\mu m$ in the plane of the sensor. 

It's about 1 pixel wide, quite lumpy, and seems to change direction a bit. It really fits the description of an ionizing particle. There are multiple possible sources, 

I'm pretty sure @JamesK has nailed it as an electron, but what else can we learn? There must be some way to get a very rough idea of how many $e^- h^+$ pairs correspond to a fully exposed pixel at "@ISO1600", then we could convert it to a dE/dx and see if it's minimum ionizing or if it's much higher.

Going diagonally, with a density of 2.3 g/cm${}^3$ that's about 1.3 mg/cm${}^2$ areal density per pixel. A [minimum ionizing particle] at 1.5 MeV / g cm${}^2$ would deposit only about 2000 eV which would make less than 1000 $e^- h^+$ pairs. That might correspond to a bright pixel. It's not out of the question that it's a minimum ionizing particle.