Question about mole and melanoma

Family member is 45 with a diagnosis of cancer (melanoma). He had several cancerous lymph nodes removed on the right and then, a year later, one cancerous nose removed from the left.

Here's the question. He lives in a very rural are so health care isn't perfect to say the least. The doctors all figured the primary site of the cancer was a mole on the right shoulder which had turned color, had an irregular shape and had "fingers" going into his shoulder when surgically removed. However the mole tested negative and now they say they have no idea where the primary site is.

1. How common is a false negative in s mole biopsy? 2. He left the mole for years without telling anyone - can cancer "leave" a mole after some time?