I haven't used the new PreCheck location in SEA, so I'm not sure about the alarms, BUT, is it possible these alarms aren't actually alarms for having too much metal, but the random selection alarm?

Since November, I've been "randomly" selected to have my hands swabbed: 4x at PDX, 5x at LAX (both at T5 and T7) and 1x at LAS. It's annoying, but so far the TSOs have all been great and it adds an extra 30 seconds or so to the process.

By the way, using PreCheck at T7 at LAX is just fine. Nobody batted an eye when I went through a few days ago. ^

Spend $50/$100 and get NEXUS/GE. It'll help your success rate exponentially and let you use PreCheck if you fly DL, AA or any other PreCheck carrier.