Triangle shaped ice/frost on the wing - anyone seen this?

Yesterday, I was flying along the tops of a broken layer and picked up
a thin layer of frost/ice on the wings with a thin triangle-shaped
pattern to the frost. This was a new one to me.

Conditions:
Plane: Cherokee 180
Altimeter 30.05
Broken layer @ 4000MSL (~3000AGL)
OAT = -5degF
Clear above the 040 BKN layer
Time was 10am, which put the sun almost directly behind the plane.

The leading edge of the wing was clear (free of frost) from the
leading edge to approximately 8 inches back (chordwise).

At the 8-inch chord mark, several frost outlines in a triangle shape
were present. The frost outline started as a point, and tapered back
evenly on either side, slowly 'enlarging' the frost pattern so that it
looked like a triangle pointed towards the leading edge. There were
several of these patterns on the wing, one every few feet (spanwise).
The frost patterns were not aligned with the rivets.

The frost triangles were about 1-2 feet long and 6 inches wide.

The OAT gauge/windshield did not accumulate any frost/ice.

Explanations? Similar occurrences?

-Nathan