Well, some sort of mystery resolved. I looked in the Orion sources and found that the buzzer stuff is defined like this:
#define buzzerOn() pinMode(SCL,OUTPUT),digitalWrite(SCL, HIGH)
#define buzzerOff() pinMode(SCL,OUTPUT),digitalWrite(SCL, LOW)
I took the board off of the XY-Plotter, looked on the back and found the SCL pad. And my program waggles it up and down as would be expected. I hoped that the other two pins indicated in the image were the buzzer, but there is no life in there.
Either the buzzer is dead, or the connections have been removed and it's there like a useless appendix...