There can be a couple different kinds of notices. One kind would be a bandwith notice, if they see your are uploading hundreds of GB per month, that could put you on their watch list and they might send you a nasty letter saying they can shut down or throttle your service. Also if you are what they consider a "power user" uploading and downloading a ton of traffic each month then they are probably going to look and see exactly what you are doing, under the initial guise of them checking to see if you are running some sort of server out of your home and that you should be on a commercial/business account which is no faster than a personal one, but they can charge you 10x as much. Worst case scenario is that your ISP actually gets a notice from the RIAA or a movie company that they have tracked media sharing back to you, in which case your ISP will be more than happy to turn over all logs that they have of your traffic, as far back as they keep them.
Gotta love the ol USofA, theres no such thing as privacy left in this country.