How often..

[Skittles]

I just have to throw my 2 cents in...

When I was a Tactical Air Controller, I had to report ALL aircraft with-in 10nm of the aircraft I was controlling. The trick is knowing if the "stranger" is going to have any affect. If not I'd say "No Factor" and this tells the pilots to not bother looking.

Oh, I forgot... I also report "not in sight" if I don't see it.