Riveria was probably making active sensor sweeps, and we know it was using its comms; it would have been relatively easy to locate then. The Repulse had probably tamped down its emissions as much as it could, running passive sensors and low-power or line-of-sight directional comms. It's a matter of how much energy you're putting out into space.
Riveria had no reason to hide, and plenty of reason not to: being a fixed installation, that has been in place for some time presumably, there are no odds in attempting to hide as everyone and their dog in the system would probably have known where to find it. Therefore it would have adopted the option of running all its active sensors to create an electronic bubble around it anyone can see (giving away information most of them probably already had) but the entering of which can be dangerous.
The Repulse, on the other hand, can move, and therefore you really do have to actually look for it. Therefore adopting a silent-running approach is a viable tactic because it doesn't have to be anywhere. It's the same reason why warships at sea sometimes opt not to run their radars, but land bases always do.