Grade II listed BT Tower to become hotel in £275 million deal - Enfield Independent

Information about it (construction, technology, capabilities) was designated an official secret for a long time, due to its importance as part of the communications network, but its existence wasn't a secret.

That has always been up in the air, certain functions of the building and it's link to the building has been designated a secret. It's not like the building itself is inconspicuous though.