Easy and simple: a small *beep* when it enters your (configurable per ship?) radar range and a *blurp* when its out of range. And of course a Button to enable/disable it :)

Edit: (finaly) Found the list with things that are already suggested. Didnt find this one but pls dont go mad if its already on the list.

check the "proximity warning" button on the bottom of the radar list screen ("u"-key)

Until the faction redux is done, there's little point in fine-tuning the prox warning to allow it to BEEP only on the entry/exit of a hostile ship, rather than any ship, since what's "hostile" is so badly borked right now.

That said, once the faction redux is done, this probably isn't a bad idea. Be nice if you could set it to BEEP only on the presence of a PC, and not any NPC, too.