Found a possible answer -

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I would bet that what you are looking at is a shrinkage cavity or the location of a trapped air bubble. The melting and pouring of metal isn't really a very scientific field. The solidification of that molten metal is. If the solidification isn't done at a very controlled rate, you will have cavities where the solidification left a void. Likewhise, especially with aluminum, if molten metal isn't properly degassed, you can leave pockets of air. If the product was then cut, it would appear as a pin hole on the surface of the piece.

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Not my words but sounds plausible...