You are correct, that looks like it is wired as a half hot switched receptacle.
Without knowing where the other ends of each wire go it is hard to say for sure. Unfortunately, I can only see the wiring in 3 of the 4 wire nuts. You have one for the Reds, Whites, and grounds. I'm guessing the 4th one is for the Blacks.
The issue I see is that in a half hot switched receptacle, the reds should not be tied together. Normally the feed cable will use the neutral and black. On the cable to the switch, the black is tied to the feed cable and the red is tied to the switch receptacle. The black and red from this cable are then connected to the switch.
While it is possible that the installer had other intentions in mind, such as having each receptacle on separate circuits, I can't confirm that and it definitely isn't how I would recommend doing it. It does make me say that you should be very careful in there since it may be fed by two breakers, one with power on the red the other on the black.
I would make a concerted effort to determine where each end of that cable go and try to determine the installers intent. Sorry I can't be of more help!
Thanks.
Perhaps I'll just avoid that outlet. I had another outlet about 4 feet away that only had 1 wire with black, white, ground that I have already converted to GFCI. I was just hoping to be able to convert and use both but maybe I'll just act like this one doesn't exist unless somebody else chimes in with a solution. I have no clue how to track down where the wire may go, everything around it, the floors above and below are finished and I checked a couple other outlets in the room and none of those have contained red. And the only switches currently in this room control the ceiling fan and light separately.
I kind of suspect at one point it was switched as the notes on the breaker box for the entire house are very detailed and seem to indicate that several outlets throughout the house are switched but since we purchased the house there isn't a single outlet we've found that actually is switched. All switches are accounted for with ceiling lights and fans and all outlets (unless there's one we've not used in the 4 years) have always been on.