... it's all very well to say they should have noticed the absence of returns beyond the rock but you are asking them to notice something is missing from a display which is far easier said than done..
No, it's easy to do, and it's essential knowledge which must be applied for proper use of the radar. I never paid any attention to this aspect of using the radar until I watched the Honeywell weather radar training video, which was a great resource for understanding what the radar was telling you.