I took a ride to Nauset Beach, part of the National Seashore, in Orleans on Cape Cod yesterday was was so surprised to see how small the beach had become after the blizzard a couple of weeks ago. You can see the high water mark and it is pretty high. There’s only a few feet of beach for people to sit on at high tide.
The dunes at Nauset Beach were pretty eroded too and the walk down to the beach by Liam’s Restaurant was surprisingly steep. It almost seems like all of the sand that eroded from Nauset Beach ended up on Coast Guard Beach a few miles away.
I took these 2 photographs looking in opposite directions on the beach. The high water mark on the first photograph is pretty high!
I think your theory about where all the sand went is a good one!