In San Francisco. Tony Bennett found it, but won't send it back. Bummer, dude...
I also lost my charger, download cable, and flash stand for my Canon 430 EX II, 600 RT. Nikon Sb700, and 910 flashes. That's a lotta losing, but I am careless when I go out to do strobist shooting.
Fortunately Promaster have come to the rescue - they have just sent us a supply of the multi-mount flash stands. These have thee shoe positions with locking holes for the standard flash hot-shoe and a 1/4" threaded hole in the underside for the light stand or tripod. it is a proper brass threaded socket too - good and tough.
Now the loss of the charger is also solvable with a Hahnel universal charger and the cable can be safely dispensed with now that I have a Hoodman RAW STEEL USB 3.0 card reader.
But it still leaves the problem of Tony Bennett...