It's PVC where it leaves the meter box, but it's copper where it
daylights and goes into the house. I'm betting its PVC from the meter
to the joint near the shut-off valve.

Prime suspect would be the coupling/adapter between
the PVC and copper. Could have a split in it.

Can you shut the water off at the meter long enough
to excavate around that likely area?

The likely area near the coupling/adapter is covered by a 4"-thick
sidewalk (they poured the slab flush against the copper pipe!). I
can't get within 6 feet of it without cutting concrete. The joint is
12 feet or so from the apparent leak, and there's no evidence of the
water coming in from under the sidewalk or from the side. It seems to
be coming up from deep in the ground (at least 14-16" below grade)
rather than from the side. I guess it could be leaking at the
coupling and flowing down along the lateral.

Anyway, I've got a trench and several potholes dug to 18" or so, and
still can't find the pipe or be sure where the leak is.

Thanks for your help, Jim.