This sequence taken on sol1972 (left navcam) between 13:27.17 and 18:26.41 local time shows the movement which I was putting down to the effect of thermal changes on either the soil or vehicle. The alternative suggested above maybe movement of the IDD or wheel tests - not sure about the activity going on through this sol on that afternoon.
Neat catch.
For what it is worth, I'd have to side with thermal changes with the whole rover / soil system. I have at work a DIY mounted webcam that I use for taking daily time-lapse movies (7s/frame at 30 fps) that faces west-northwest. As a result of being very solidly secured in a sturdy cardboard box that itself is very firmly wedged between a large window and an institutional 'radiator', thermal expansion and contraction movements are rather visible through the day, afternoon and evening in particular. I say all of this because the *smooth movement over time* that is shown in the navcam animation is VERY reminiscent of the thermally driven *smooth movements over time* in my own time lapse camera, and at a time when such movements seem likely (the setting of the sun). An argument from a very imperfect analogy sure, but still I feel rather compelling.
-- Pertinax