Russian vessels of all kinds are in the gulf & carribean all the time. Most we know about. I suspect a few, sub mostly, slip in without us noticing. I wonder how they found out about it, if it happened.
I wonder if this is SOP, to let them tinker around thinking that they're "undetected" (and they do that for us too). I mean otherwise we'd know that they knew what we think they don't know. It's a great game, why ruin it.
I think the "shock" of finding a sub in the gulf, that they probably knew was there all along, is more to influence the DOD budget #'s than it being an issue of national security.