You can modulate an LED at a very high frequency and with the correct receiver send data[1] theoretically you could put a passive receiver into a light fitting (or something as low tech as a high gain microphone), have it record data and then play it back via the LED's, someone walking through the building with the correct receiver could then download the data.
It's a long shot but intelligence services have done strange things (fake rocks, bugging devices in typewriters, setting up cameras over copy machines etc etc)