This is just a problem that DW has with the way it models active sensors. There is a detection threshold, which is a discrete range at a given aspect. At that threshold, the contact is theoretically detectable and produces an audio return. But, for the visual representation, there is a brightness scale going from strong to weak that doesn't even begin to become visible until the target is well within the detection threshold. It's not your eyes, it's just the way the interface/model is set up.
It's actually the same way with the radar model--but without the audio cue.
You don't see it yet because all the databases out there (maybe RA is an exception) go from the detection threshold to a very strong contact in very, very little space, so you never have to deal with the "invisible" returns.