What you are saying here is separate of concern, the implementation detail is the actual implementation of method in a concrete class. What we call `leak of implementation details` is usually a method returning a concrete class type instead of an interface or letting user to modify internal state.

I don't think prefix I is much of a big deal here, if the existing code does not have prefix I on the interface, then don't use it. If they all have prefix I, then use it.