I am just another Google Nest customer observing that it may not be fixable. Rain setting off motion detectors of all brands and makes is a common problem, especially at night.
You could try lowering the "Wake-up sensitivity" to "Low", but then your camera might not pick up other events, especially at night.
Sorry to hear you're experiencing this, as I can imagine it has been an inconvenience. I would love to look into this further for you. Thank you so much to, MplsCustomer for your help and insight on this thread. I would agree, there isn't much you can do to keep that from happening, but you can adjust the same settings as those that were mentioned on this thread.
Just checking in to see how the conversation is going. Please let us know if you have any further questions or concerns. We'll leave this thread open for a little longer for follow-up questions and concerns.