Electrical – Locating HVAC Breaker To Replace Thermostat

I'm looking to install a smart thermostat in a single family home built in 1997 (southern California). My issue is that I cant seem to locate the breaker for the HVAC equipment. I was only able to find a single panel for the house and there is a 50A 2-pole switch labeled 'A/C', but after shutting off that circuit I can still run the air conditioner and heater (I'm not 100% certain the air conditioner was still pumping out cold air as I didn't wait long enough, but there was a draft on the vents. When I ran the heater I could again feel the draft and smell the hot air). The only other 2-pole switches are labeled dryer and oven (30A and 20A respectively; and I'm inclined to believe them but I didn't test with those breakers switched off yet). I noticed there is a 'pullout disconnect' on the outdoor AC unit, but my understanding is that is just a manual disconnect for servicing and isn't meant to be a breaker for the circuit? (if you haven't noticed yet, I'm not all that knowledgeable when it comes to this stuff). There appears to be another panel located just under the gas meter (or as part of the gas meter) that's adjacent to the breaker panel, but it's locked (although the lock doesn't look official – I may just cut the lock to see what's behind the panel). I was hoping to avoid flipping the breakers blindly, and I'd be surprised if the HVAC equipment was running on one of the other breakers as they don't seem to have the capacity.

I'll mention that the thermostat wiring has a jumper between Rc and Rh (C, W, Y, and G all wired). I think there is a backup battery in the existing thermostat but my understanding is that all components will receive power from the transformer that's feeding Rh, which should correspond to a switch on the breaker (or is it universal that Rh is fed from the HVAC equipment running on a higher capacity circuit)? I'm guessing that's just a control circuit so it doesn't need to draw 240V@50A and so it's probably running through one of the smaller 1-pole breakers? Maybe the AC unit was disabled when I flipped the switch but the control circuit, fan and heater are running through different breakers? If this is the case, I'd only need to find the breaker responsible for Rh i.e. with Rh off, I can safely swap out the thermostat?

Any thoughts/comments/suggestions would be greatly appreciated.

Thanks!