Romans commentary:

The Virgin Birth is found in strange places and unique locations. While not completely apparent in Romans 1:3, the "Seed of David" was considered by early Christians as referring to Mary (See also Galatians 4:4-5). In every reference in Scripture, Mary is mentioned in the birth of Jesus, not Joseph. Every Christian would have thought this strange unless there is another doctrine that they also believed, like the Virgin Birth. Add in the virgin Birth and it makes sense to refer to Mary.

However, Romans 1 is a little more opaque than the explicit references to Mary. Irenaeus believed Romans 1:3 referred to Mary as noted in this quote, "one God, who did by the prophets make promise of the Son, and one Jesus Christ our Lord, who was of the seed of David according to his birth from Mary and that Jesus Christ was appointed the Son of God with power, according to the Spirit of Holiness, by the resurrection from the dead, as being the first begotten in all the creation; the Son of God being made the Son of man"

It is likely that the Christian Church read Romans 1:3 as referring to the Davidic Covenant, but also to Mary and the virgin conception.

Implications? Doctrine does not always have to be explicit to be present. The Church could read a text and understand the doctrine it is alluding to without explicit reference to that doctrine.