Hi Angelo, I do not know the percentage of people that "show no signs of melonoma on there skin but have it internal."
I had lesions that showed themselves under my right thumb nail ten years ago. They removed the nail and nail bed and said have a good life.
Ten years later, last Dec I noticed a lump under my right arm pit.
Biopsy proved dead cells there. Cat scan showed trouble on my liver, biopsy there proved "matastatic malignant melonoma".
Brain scan proved I had two tumors there. Gamma knife took care of them, but won't get another scan of the brain till this June.
Currently I'm taking Immunotherapy using Opdivo and Yervoi.
So far dr said after two infusions my numbers have come way down and my body is fighting very hard for me.
First couple of months after diagnosis were pretty dark for us but emotionlly I'm feeling better and stronger. The numbers helped a lot!
My son bought us all Pac Man T shirts to where every time I go for infusion. The nurses love them!

My initial diagnosis was due to an external tumor (a mole had begun growing over the course of a year before I acted) but ALL 12+ subsequent tumors (including in lung, pancreas, liver, kidney, pectoral, armpit and lymph nodes in numerous places) were internal and undetectable except by CT or PET scanning - I never felt anything wrong. It is often a silent killer. Quarterly scans for over 20 years literally saved my life numerous times.