Answer:
The sentence's complete linking verb is "might have been."
Step-by-step explanation:
A linking verb reveals a relationship between the subject and the complement, that is the part of the sentence that follows the verb, because it provides information about that subject. Unlike other verbs, linking verbs carry no action: they simply link the subject to its complement.
Some of the most common examples of linking verbs are to become, to appear, to seem, or to be. This last verb is the one used in the sentence, and its full form in this case uses a modal verb -might- and the present perfect form.