I run TMO and can pick props up on sonar at slow speeds of my own sub.
You can get a lock with the green light and not be able to hear the ship if they're far away unless you crank sound way up. Sonar will also lock on sunken ships, small land masses, ect., causing the light to come on.
My concern is you're not picking up your own prop noise. Could be a RFB thing. Did you listen at a higher speed?
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