These files being requested are not about current investigations. Anything that is a current investigation is not covered in FOIA/Privacy Act.
So even if you were to start to be investigated as a result of the request (which is illegal for the investigatory side to know what requests were made, though obviously that is self-regulation they have to abide by), it wouldn't even show up in a second request unless you were completely exonerated.
Given they abide by the requirements it makes sense the way they set this up. Say they are investigating a serial killer or terrorist, he FOIA requests himself, knows that they know about his method of hiding his tracks and changes it up as a result of the request.
Based on people I know who have done this and my own requests, if you've ever received a cease and desist over torrenting it's guaranteed to be in there. Also, there are going to be instances of torrenting that you have done that you never received a cease and desist over. Known "groups" you are affiliated with are on there, and any correspondence you've had with federal agencies as well as state ones, such as driver's license records, known traffic violations, IRS tax records, car purchases, known residences, etc.
Do you have an FBI file?
You might! Many people do. So let's talk about your past. Did you ...
The repeated "I think I'll be on some sort of list for saying this!" mentality is quite absurd, really. Their "list" is the entire list of citizens of the country and what they are suspected of and what they are known to have done. And really, this makes sense as well if you think about it, they have the DMV records, the birth certificates, etc of the entire population, what did anyone expect the world of computers would bring them? Did you think they wouldn't want to have a record of your prior known violations or pictures of your driver's license and descriptions of your person in the instance they now have to form a manhunt on you since you murdered 20 people? Do you expect them to petition your local government agency to get these records when half of them your neighbor can look up on the internet in minutes?
I'm not saying it's good, and I'm not saying it's bad. I'm just saying that it's the way it is and that it was inevitable. Everyone wondering "if" they have an FBI file is kind of naive to think that the government doesn't keep track of it's own records.