This won't cover a square light-year, but the Romulans are too stupid to fly around.
A tachyon net
is a system developed by the Federation
to detect cloaked starships
attempting to move undetected through a specific area of space. The net requires a fleet of starships, space stations, satellites, or other platforms to transmit beams of tachyons to each other. If the beams are close enough together, a cloaked ship attempting to pass through the net will interrupt one of them, alerting the fleet that something is attempting to slip through the net.
A tachyon net requires a large number of transmission sources in order to effectively cover a significant area of space.