What might cause mild, intermittent right flank pain in otherwise healthy, asymptomatic patient?

Many things: Muscle spasm, kidney stone, posterior rib malalignment, facet joint inflammation/injury/degeneration to name a few. Best to see your doctor if it continues and have this checked out. Hope this helps. Best of luck. Dr R

Flank pain: could be from a radiculopathy in the distribution of a nerve in that area, from the underlying muscles or from the kidneys. The latter often radiates down to the groin and/or the testicles. check for evidence of stones in the kidney or ureter. An ultrasound of the kidneys will rule out some of these issues