armatata,
Here's my experience running TMO, RSRDC, and a few others.
If the green light comes on, there is a ship that way. If it doesn't, there is not. (Keep in mind, the green light between 170 and 190 is your ship.) That's the only way I've found the sonar to detect ships. As for radar; that's a bit more sporadic with its detection. However, signals detected are always bright. There will not be any slight changes on the radar screen either.
My advice: Just do more patrolling. Try to find natural choke points and position yourself directly between ports that will probably have traffic.
Fair winds and following seas,
SilentPrey
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