They look like the specialized anchoring structures produced by a climbing vine to remain attached to a vertical surface.  The anchoring structures are specialized roots with the root hairs secreting a kind of glue that can be stubborn to remove.  The two most common culprits are English ivy and Boston ivy.  "Ivy League" refers to the vines covering campus buildings.

Home inspectors don't like to see ivy growing on homes owing to potential damage such as the loosening of brick mortar or in your case, the ivy may grow under the edges of the siding to loosen the siding's attachment points.  My guess is the previous owner removed the ivy before selling.  However, the anchoring structures remain behind.

Unfortunately, I don't know of any chemical that can be applied to dissolve the bio-glue and intense scrubbing may damage the siding.  You may want to seek the advice of a siding expert so the cure isn't worse than the affliction.