Not to muddy the waters but the Supreme Court has given some guidance on this too in Florida v Bostick. The case related to a consent search in the confines of a bus but the standard has been used in other cases dealing with consensual(casual) encounters, especially those with officers walking/driving up to subjects. Note that it is not based solely on the subject being "free to go".
In this case it would depend on the OP feeling free to decline the officer's demand, or show of authority, to see his identification. I would assume the OP did not feel free to decline so it would then convert the encounter into a Fourth Amendment seizure regardless of whether he felt "free to leave" or not. Since the officer already had PC to detain him the argument is moot though.One must not look at whether a party felt "free to leave," but whether a party felt free to decline or terminate the search encounter. The Court held that in the absence of intimidation or harassment, Bostick could have refused the search request. Moreover, the fact that he knew the search would produce contraband had no bearing on whether his consent was voluntarily obtained. The test of whether a "reasonable person" felt free to decline or terminate a search presupposes his or her innocence.