I never saw my mucus plug with my first, my labor started when my water broke spontaneously. Maybe I lost it in the shower or something, maybe I lost it while on the bed in the birthing center. No idea. Not a clue.
With my first, I didn't loose my mucous plug till I was 3cm and at the hospital already in labor
You can start getting contractions and such before hand, but usually you do lose the mucus plus, just some people lose it without noticing. My sister didn't notice hers at all, but the doctor also had to break her water for her.
It usually comes out before you go into labor it comes out when you dilate