I have the same fear as well. After going through the precheck WTMD at MDW yesterday, I was bothered that it took the person behind me in line so long before going through the WTMD with the fear that the beep would come about during the time that it took him to do whatever he was doing before going through. Fortunately for me, the beep came through during or after he went through and that person was selected for the body scanner (which I would have opted out of) instead of me.

In the future, I may try to wait to go through until there is another person right behind me as it seems that this will lessen the odds that I am selected.