I think that assumption is true. We half joked about sweeping the White House for bugs initially, but there is definitely something going on.
I would have every square inch of the Oval Office and any rooms where sensitive material was discussed swept both electronically and physically; the bug will have to have power so any place where there are electrical wires should be checked. An old favorite was the electrical socket with both built in camera and microphone. Stays on 24/7 and easy to install.