First thing I'd do is remove that wall plate and look at the back - to see which connections are actually connected to wires. That is actually a 6.2 wall plate - there is only one "Rear Surround" input. I also suspect that the "Left - Surround - Right" are actually connected to the speakers in the back - most people don't know that the Surrounds actually go to the sides.
The subwoofer input(s) may not even be connected to a wire. If one of them is, maybe you can see which direction it is headed. In-wall subs are rare, more commonly an RCA cable for a powered sub would just lead to another wall plate somewhere else in the room like Arny suggested.
2-Ch (HT L/R): Oppo BDP-105 BD, Adcom GFP-750 pre, Bryston 10B Sub Xover, Bryston 4BSST2 / Paradigm Signature S4 v.2 (L/R), (2) SVS SB12-NSD (Subs)
Home Theater: Bryston 4BSST2 amp / Paradigm CC-590 (C), Outlaw 7700 amp / (4) Def Tech UIW-RSSII (LS/RS/LB/RB), Samsung 46” 3D LCD