Exploratorium image of 'string' trail

We've seen this a few times now...I may go back to find the other images...I think it started with Oppy. Most likely the dust is falling into cracks along the edge of rocks or slabs beneath.

Since we'll be in the area for some while, maybe we can watch to see the dust fill in or the gap get larger.

[edited after further info found]

Navcam exposes process...