+8 sounds right. Remember that at this stage it’s just about detecting, not identifying or getting detailed readings.
A ship using active jammers sends out massive amounts of signals to overwhelm enemy sensors, so it should be simple to see that something is out there, but it might be very difficult to see what it is.
Compare it to trying to spot someone outside in darkness, with your eyes. If the target aims a flashlight directly at you you will definitely notice that someone’s out there, even if you can’t see exactly who or where.