If you noticed there are some circles around the DDs in the graph. I haven't made that post so I don't want to talk about it but if I speculate I will say that they indicate the minimum detection range of the DDs. What do I mean with that:
The fact that if you're 5Km away they don't see you doesn't mean that if you come closer they will automatically see you. So you can actually close in to very small distances without being seen. It takes a bit of experience and loads of luck to perfect the move but it can be done. That's why I talked about weather conditions. If the weather is favorable (heavy seas, night and a light fog) you can approach far closer that you would in calm seas conditions. If the fog is heavy you're basically as blind as they are.
Does this make sence? If not tell me and I will elaborate more
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