If she'd just finished an exercise she might well have remained in total EMCON so no radar, just navigating visually.
Do Navies do CRM anything like we do? In a watch change situation perhaps with poor designation of tasks between the watches and no one head down on the image intensifier screen, visual nav being the odd glance out of the window by 'somebody' a big, black tanker might well not be spotted. Even with radar in use if 'nobody' is watchng it because they're busy briefing 'somebody' - and all jolly and relaxed at the end of an exercise...
Penny to a pinch of the proverbial this'll turn out to be a major CRM clusterboob.