I'll say it a 3rd time, it has nothing to do with the WiFi connection. It's a Geofence. The car has the coordinates of all the Tesla facilities I listed in my previous posts, and if you get within range of that location, the car sends a message to the mothership and if they decide you are ready for an update, one is created for your car (a patch). This patch is relatively small as it only contains the differences from the new version from your existing version. (Usually well under 100 megs). The patch usually starts downloading within a few minutes, and will download over cellular or WiFi, and if the connection is lost or changed, it will resume where it left off. The download usually only takes a few minutes even on 3G. Once the patch is downloaded it's decoded and verified. This process takes the longest and will usually be evident as the CID will be laggy and slow to respond, as there are significant computing resources needed. Do not reboot your CID if it gets laggy during this process our you risk killing your update.
One the update passes verification, that's when you'll get the notification and Alarm Clock icon.
I do not yet know how the mothership decides when you are eligible for an update, but it's clear to me that it's very rare to keep getting them one after another even if you regularly visit a Tesla facility. There are enough exceptions to keep one guessing. Maybe that's by design.
I would not advise anyone to bother driving to a Tesla facility to get "the latest". Maybe once 8.0 is released and there are major changes, but otherwise you are wasting your time. Most of the stuff released are just small bug fixes. It's very unlikely you'll get any cool new must-have features to make it worth your time. Relax, Enjoy your amazing Tesla!