Unfortunately having pre-check doesn't mean you are guaranteed to not have to opt out from going through a scanner, since you could be randomly selected for MMW with the beep (unless this has changed recently). Is there any official policy about the random alarming in the pre check lines? It seems different from airport to airport. I went through one recently where the pre-check line only had a wtmd, with no scanner nearby and a wall seperating it from the main line, and I didn't see anyone trigger the beep. Then other airports in the past it seems like 25% or more of people trigger it.