I'm restarting this question from My watch thinks I'm asking it something. … - Apple Community because the fault is unresolved.
When I have my AirPods in and someone walks up to me and starts talking Siri randomly starts speaking nonsense from my Watch. The apparent fix is to turn off "Listen for Hey Siri" and "Raise to Speak" in the Watch app.
The problem is distinctly related to wearing the AirPods at the same time. It occurs with Airpods 2 and 3.
It's a shame that these great features just don't work and that apparent workarounds require one to deactivate feautres that are supposed to just work.