forkandles

Have just heard on the news that an historic bridge by Brunel has been discovered just outside Paddington station, encased within a bridge built later & was nearly destroyed along with the bridge surrounding it. Ok so it wasn't visible, but how can you lose a bridge? .. "Hey has anyone seen our bridge? It was here last time we looked" ..
If a bulding or major construction is demolished records are kept; surely the absence of such a record should have been some kind of clue??