Info I have says B codes are set by Radio, Instruments and BCM. Closest match I can find is B1342 Rear left speaker open. Do you have rear speakers set up? If so, there might be a loose wire connection, either at the speaker or at the head unit.
Sometimes it would be nice if you provided a little more info like the year of the bike and mods that you had done to the bike. B codes are related to the audio system and since the left rear speaker is out it would be a good place to start.
Assuming you have a 2015 or newer RG, CANBUS diagnostics are very sophisticated. The computer literally knows all. It monitors every light bulb, every sensor, practically everything. You answered your own question - you took the speaker out and must have turned the ignition switch on, the system detected a problem and alerted you to it.
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