I work with a retired Cold War submariner, and asked him about the “fast movers” or the fact that we track them. This is what he told me:

So a bit of a small preface, we became friends at work because we were both on submarines in the past, but I worked in the engine room. Pretty far away from the world of tracking sonar contacts…

So anyway I asked him after reading a post on here about how the government tracks fast movers he worked in the sonar room, so he was the guy to ask about it.

He said that they would put all “unidentified” contacts (as in clearly not biological – there’s a lot of unidentified biological noise) in the unidentified section of this logbook style thing they had. It was like a file with all the navy’s info on how other ships sound. He said there were some entries they made which were really weird, either very fast with nothing in the area that could’ve made that noise or just weird as in strange noises like buzzing or something that’s clearly not natural but sounds like no ship they’ve ever heard.

I guess that file and all the recordings would be sent off the boat periodically. No word what happened with it after that.

Edit: edited to add the fact that he was a sonarman so tracking things was literally his job.