Not in the article, but the implication from this study is that it is surprisingly easy to "bug" a room using purely opt-mechanical devices and keep all detectable electronics far removed.

For example, single strand of very thin glass fiber that can't easily be detected by an eye and receiver several rooms away or even hundreds of meters away.

To protect against laying out thin fiber inside concrete or even under the paint beforehand would make eavesdropping undetectable. No wonder they watch every phase of embassy construction so carefully.