UPDATE: Resolved. The ground wire was located behind the box, in the wall. It's a bit short (??) but it will do once I put the subpanel in. Thanks for everyone who responded.
I am tracing the somewhat old wiring network in my house... and I found one blind box on the patio (outside), with 3 8AWG wires inside, two hot and one neutral, that is connected, via a 50-60' wall run, to my main panel and a pair of 60AMP breakers.
I am guessing that was for an AC, since that would justify using 8AWG wire on such large breakers. Now, I don't have an AC there, or a need for it, but I could use a couple of patio outlets.
I was first thinking about putting a small sub-panel there, to have some space for future expansion, but digging into the wall is out of question, so I cannot put the 4th wire (ground) in to make it code-legal.
So, in fact, with 3 wires only I seem to be limited to doing a 120V setup? Can I just install a smaller single breaker at the main panel, say 20A, and then use the 3 existing wires as hot/neutral/ground (re-wiring them at the panel, obviously). Do I need to mark them, e.g. "turn red into green"? Or is there a more interesting way to use the existing wiring? What would you do? ;)