I've seen this a few times when resist overloads
the developer and panels exit the developer with this "foreign body/contaminate"
attached, if it is not cleaned prior to plating it inhibits copper buildup
there, then during rinsing or some point after plating prior to tin plate it
comes free...leaving a nice little depression in the copper.
As for where this might be mentioned within any
specification, I have yet to find this particular condition so I treat it as
trace reduction-height, and/or poor workmanship.
Depending upon the customers use of the board, the
type signal running through the trace, and if they're in a good mood that day,
this may or may not be considered a reject...but it is something that should be
brought to their attention...