B2784 Entry and start/smart key

When taking my Prius in for a smog the other day, I got a B2784 code on the OBD scanner. I replaced the start button, but this hasn't gotten rid of the code. I checked all the wires plugged into the Immobilizer ECU (blue box behind the glove box) but they all seem fine. I checked 12v battery and it's in good shape. Disconnected and reconnected the 12v battery and cleared codes on the OBD scanner and the B2784 code is still coming back. Anyone else had experience with this issue? I would appreciate any suggestions for what to try next.
One other question: does your Prius give one single beep every time you turn it on? When I hit the power button it beeps once when going into ready mode. I'm second guessing myself on whether it had always done this or if it correlates with the B2784 code appearing.