Well I think that's when you have to say "No, [pointing] *that's* the PreCheck line, this is non-Precheck priority as per the sign" and be insistent and just move ahead. But just be sure because TSA won't let you through precheck if you aren't eligible, lol. My last time through a couple months ago it was such a scrum I couldn't tell if there was a non-Pre priority line, I just saw the (empty!) pre-check area and made a beeline.