The radio signals most likely to be detectable would be those emitted by scientific probes or even communications between space ships (for space traveling civilizations).
Such signals would of necessity have more power behind them and thus be more likely to be detectable over long distances. Normal intraplanatary communications only need sufficient power to travel at most a distance measured in kilometers. Communications to distant probes need power and frequencies that can travel gigameters or more. Other civilizations might conceivably need to communicate with probes over a light year or more in distance.