Rights Exist Outside Law
Rights are outside legal authority and, therefore, need not be "invoked" in order to actively exist in all situations.
There is no need to "invoke" the right to remain silent. That part of Miranda which deals with this issue is enough for the officer to know that the right exists, although they should know it without, due to the fact that they enforce law, which merely acknowledges the existence of all rights and has no authority over them.
Rights are always in effect like a legal opt-out program, and they need not be invoked like a legal opt-in program, because they exist outside law.