First lesson, which you have no doubt now taken to heart, is to keep your BP in your hand or pocket during the security process and NEVER surrender it (there is no reason for the TSA to take it from you).

SEA is an enigma. Once last month I was the only one in line at the C checkpoint ! Still got a random (make that punishment) secondary of course. At other times it has been up to 30 minutes, so I usually eyeball the line and if it looks excessive switch to the S terminal train and catch the Horizon bus back to C airside. It all depends on the time of day.