I had this quite early with my daughter and have had some but not as much this pregnancy. It doesn’t actually mean that baby is coming early. As our mucus plug can regenerate it’s actually really common to lose some before. However, as you’ve had two early babies, I would definitely give triage a call and ask to be seen to check for dilation etc too x
I'd get it checked out. I lost some of my plug in my last pregnancy and went onto labour. I didn't seek advice as had read others'stories of it not being an issue. I'm sure this isn't what's happening with you, but hopefully shows the importance of getting checked over.
In one of my antenatal classes, they said that if we lose the mucus plug, we should call the antenatal clinic because labour might start soon. I'd say you need to call the hospital and let them know