Firmware Update? (Answer: No)

I really thought we were getting a FW update yesterday. While my G1 was syncing to Garmin Connect Mobile, GCM said it was also transferring a software update. Once it was done, on my G1 I went to System>Software Update and sure enough, it said Update Available. I selected this, and just got a screen which said "TZ" with a down arrow. I pressed the down button, and the firmware update progress bar appeared, ran for less than a minute and then the watch just returned to normal, still running 6.1.6. I tried rebooting to see if that triggered anything, but no - and now Software Update doesn't show anything available.

Anyone any idea was this was? I can only think of TZ as standing for "Training Zones", but those just sync through GC, they don't get transferred as a software update.