not seen this in a while, it looks to be the pinhole leaks that we used to get in the 80’s italian copper tube (i think its italian) – the tube had slight internal flaws that left raised bits, so, like a pebble in a river, the flow dug a hole after the raised projection. The flow must roll over the rock, then down and back on itself. (I could do a somewhat better description, but it might get quite boring).

Interestingly, this section has been like this for quite some years.