I just want to thank everyone for their kind thoughts and comments about our missing cat. There is still no sign of him anywhere, and at this point, we suspect that he purposely went away, sensing it was time to "go".
I've heard that cats like him (with outdoor access and most likely born to a feral mother) will do this when it's time. He wasn't displaying any obvious signs of illness, but he was around 13 years old and had a heart murmur.
I do have fantasies of him showing up like always at the back door, but I don't actually believe that will happen.
There is some solace in the thought that he went out on his own terms, but I just didn't know that the last time I saw him would be the last time I saw him...