It would be nice to know when for instance turning your aircraft away from a target will result in that target being lost. Sometimes it's obvious, sometimes it's not!
It would also be nice to know when your radar is actually doing anything; I think it's quite often that your radar is doing work for yourself and for the team but it's not obvious when that is.
I think this would teach players how effective their sensors are in various situations so that they better know what to expect from them, and also what it means when they don't detect anything!
The symbol would only need to be something minimal, like an extra line or dot somewhere on each target marker.