After this encounter, what is Utterson's concern regarding Dr. Jekyll?

Utterson is afraid that something in Jekyll's past has come back to haunt him. He is afraid for his friend and attempting to figure out where Hyde fits in, and what type of hold he has over him.

And the lawyer set out homeward with a very heavy heart. "Poor Harry Jekyll," he thought, "my mind misgives me he is in deep waters!  He was wild when he was young; a long while ago to be sure; but in the law of God, there is no statute of limitations.  Ay, it must be that; the ghost of some old sin, the cancer of some concealed disgrace:  punishment coming, PEDE CLAUDO, years after memory has forgotten and self-love condoned the fault."