I almost walked by...but then I stopped and gently probed one end of the trail.
Here's another clue. It's an extreme closeup taken under the microscope:
Those black spots with gold flecks are eyes looking to the left. (But the eyes look different at this stage than they do later in the life of this animal.)
This next image will give it away:
The trail was made by a tiny Mole Crab (Emerita analoga)! There were several of them scattered along the beach on 24 February 2014. These crabs had a carapace length of only ~4-5 mm, which means they settled out from the plankton not too long ago.
Here's one more view (below). Watch for these meandering trails and new Mole Crab recruits at a sandy beach near you!
P.S. For a picture of an adult Mole Crab and a little more information about them, see the post from 25 February 2012.