OK, I was a bit skeptical of this SC/firmware update connection as well, but here's the same story as others have told:
1. Been on 2.20.30 for over 5 weeks (received the update over my WiFi at home on 5/27/16, installed it on 5/28/2016). I have a massively powerful WiFi access point at home, and I get 5 lines (full signal strength) in the car when its in the garage. It's connected to it every night.
2. Have not been to a service center from that date until now.
3. Stopped by the Houston-North Fwy service center today to supercharge. (Supercharger, Gallery, and Service Center is all together).
4. Checked the WiFi when I got into the supercharger stall. The only one listed was "Tesla Guest" and it was password-protected, so I didn't connect to it.
5. Supercharged for 20 minutes.
6. I was busy taking a walk, so I didn't get back to the car until 10 minutes after the supercharge finished, total 30 minutes in the supercharger stall.
7. Checked WiFi when I got back in the car, it was connected to "Tesla Service", which had not appeared earlier. However, the signal strength was very low, I'm not sure it really had a good connection.
8. I disconnected from the supercharger and drove out of the parking lot.
9. Literally 60 seconds after I left the parking lot, the software update icon appeared on the IC. I had just completed the U-Turn under the North Freeway to head south.
I make no claims that the WiFi has anything to do with it, nor do I have any evidence that geofencing is responsible. But if this is simply a coincidence and has nothing to do with the service center visit, then it's an extraordinary one. There have been 14 other firmware builds since 2.20.30, starting with 2.20.45 on 5/28, and I haven't received a single one.
It's been 36 days since I installed 2.20.30, which is 51,840 minutes. I received this new update (installing now, so I don't know what build yet) on minute 51,841, after visiting a service center that I never go to or drive by.
Believe what you will.
Edit: Install completed, it's build 2.24.102.