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You can Relax :) It is with 100% certainty small tracks of corrosion which follow a path of least resistance under the paint. If you were to scrape the paint away you would find a track of surface corrosion but nothing of any structural consequence.. I have had it on countless frames and it can be lived with until such time you feel a trip to the painters is needed.
Put a dab of Kurust on any exposed surface oxidisation when you get a chance or before touch up. - This actually converts corrosion into a hard substrate.