If you're seeing icons on the NavMap turn on, then about 30 seconds later, turn off, then about another minute and a half to five minutes turn on, that would either be the radar "sweep" if outside visual range, or if at the fringe of visual range, the crew gaining and then losing identifiable visual contact. In either case, you would probably see sonar lines pop on and off also as visual or radar contact is lost and regained. If you're seeing them further out while at higher TC, those are the radio reports coming in that turns them on, and then the "decay" time after the report turning them off...
