I don't know about now (as I've been out a few years), but, when I was in the Marine Corps Reserve (before the digital camoflauge came out), no one in the Marine Corps had patches of any kind on the sleeves of the utility uniform. Part of the reason for this was to make it harder for the enemy to determine which unit they might be observing. The other reason, as it was explained to me, was that there is ONE Marine Corps, regardless of their MOS.
Having the flag on the shoulder may be different now, as this would be for everyone uniformly, but we did not have it back then.
Wearing the utility uniform off base, however, was a different story entirely. This was allowed off base ONLY when enroute from your place of residence to your duty station.
If this individual was simply hanging out trying to look cool and did not appear to be on their way to some destination, then chances are that your suspicions are right. If they claimed to be a "Sniper in Force Recon", then your b-s-o-meter should really start going off.