Well all the research that I have done the mucus plug is usually at the first stage of labour and next comes the contractions and then the birth of the kittens. As all stages of Cat Labour I have looked into have said the above I would suggest contacting your local vet on the phone and asking for their professional opinion. Good sign your cat is not in pain. Also are you sure the kittens definitely havent been born somewhere else that you are unaware of?