I believe it was holes for termite treatment.

Can probably explain how they just became visible

So many years ago they were drilled, and patched. No the patches age
and detoriate. Eventually they are ready to fall apart when a heavy
rain storm comes from a odd direction and washes them away. or someone
sprayed the area with a garden hose.

this explains the particles around the holes, left overs from the
washed out patch material.

so wash the area and holes with a garden hose, and while area is nice
and moist use plain old mortar mix, it will start out lighter but
darken with age.

I am 50, lived here since I was 12.

There are repair projects I have done here I have totally forgotten
about over all this time, life is busy.

termite job was probably his parents.