I apologize for not getting back to you sooner.
I agree that his behavior is not appropriate. Not necessarily because he is a sex predator. I do not know if you are implying that he is one, or if you are asking what is a sex predator's behavior patterns.
This link gives a good description of the what behaviors are evident in child molestation.
i think it is inappropriate because it is behavior that does not respect boundaries and provides opportunity for inappropriate touching to occur. It can send the wrong signals to the children and to others who are observing.
We did not establish how old Grandpa was. BUT, I can say, in working with older people, (68 years and older generally but can occur younger), I find that they are sometimes in a state of denial of their aging. This shows up, sometimes, in overly affectionate contact with youth, defined as anyone 20 years or more younger than them. In many instances they will be trying to hug, touch, kiss, and with men, even use sexual innuendo as if they were trying to pick up the young women. They can actually sometimes get quite assertive. This is all related to denying their aging. Of course, those who have come to terms, and age gracefully, do not have that problem.
This does not excuse this behavior.
You need to set clear boundaries and enforce them. If he is an older grandfather, then perhaps substituting a pet may help to divert his behavior.
I agree that the potential for inappropriate touching is there.