It is possible that the previous owner flashed firmware that doesn't bind that that key to anything. So the first thing to try would be flashing firmware that does bind the key. If that doesn't work, the switch might be the problem. You could check on the back of the pcb if the soldering looks any different from the other keys. Even if it doesn't you could reflow the key. If that doesn't work you can unsolder the key and pull it out, open it up to see if anything is messed up like the contact leaf. You could try a different switch in that spot. If you put in a fresh key that works in the old spot and it still doesn't work it might be the pcb. Maybe you need to reflow or replace the diode. If that doesn't work it might be the 'wire' that is etched on the pcb. You could jump it but that really is a pain in the ass. I have no experience with that. Good luck.
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Before opening up or resoldering any switches, I'd short the two pins with something (tweasers or similar) to confirm or rule out the switch itself as the cause.
Thank you so much for the thorough response!
It seems it's easier to deal with the firmware from a Windows machine, so I'll deal with that back in the office next week.
If that doesn't help I'll take it out of the case again and see what's up. I don't have a soldering iron yet, but if it's an obvious problem this might be what pushes me over the edge. :)