https://en.wikipedia.org/wiki/The_Thing_%28listening_device%...
Imagine something like 'The Thing' but with ~75 years of technological advancement.
The Capitol is going to need to be cleaned for such devices and equipment for a long time before it can be considered secure again.
On the flip side, any devices that may be found are likely to be close to the latest models, and like with project SATYR, the US may have a potential goldmine of new tech in the coming years.
EDIT: Combined with the recent hacking of the US, the synergy of having physical access creates a load of headaches and nightmares. If I were in the federal information security space I would be very interested in visa and flight logs in and out of the US right now.
> I've also heard that the congressional paging system locks devices when an emergency is announced
If computers locked automatically when an emergency is announced, it doesn't matter if the staffers evacuated quickly.