"... had touched her hand" if the contact was before the moment under discussion. Example: "After the meeting, she told me not to touch her again. I didn't know that I had touched her hand." "... was touching her hand" if the contact was at the moment under discussion. Example: "During the meeting, she told me not to touch her again. I didn't know that I was touching her hand." "... has touched" is incorrect because "I didn't know" refers to a past moment.

Thank you

I interpret them to mean different things. One talks about the event as it happened in the past. One talks about the event as it's still happening.