Yes, judging from the fact that you asked about it, I thought it would be actually the subject's finger. What I meant is to say it's unusual. One of the reasons might be that it's hard to add 指 there. There is no problem with 唇に触れていた. 指で唇に触れていた is acceptable but sounds too objective. Anyway, my impression is that the writer seems to have a tendency to prefer to use causative forms.