Again, for clarity: NOT the Sonoff RF Bridge
The button on the SonoffRF Basic is not on GPIO0 … At least I can’t get a response from GPIO0 in the code, and the button does not put the Sonoff into the flash mode.
Any ideas?
You mean this one?
So you anwsered your question already 
Often the RF modules have there own brains an that button is wired to the RF module (to pair/toggle) and not to a wifi module (maybe a esp) which is also inside the box 
No. That is the Sonoff Basic. I have a dozen of those scattered all over the house. My question regards the Sonoff RFR2. GPIO0 is on the RF board pins so flashing is just a little more difficult than the Basic where you just hold the button down to ground GPIO0.
I did, however, finally find the schematics (page 14). As @orange-assistant said, the button is connected directly to the RF transceiver- not to a GPIO pin.
But. Since I am not using the RF module, I can jumper the KEY pin to the GPIO2 pad and use the button on the module for on/off.
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