"Be careful with this message. The sender's account may be compromised, so this message could be a scam to steal personal information."
When I clicked on the "Learn More" link, the explanation said:
"Why am I seeing this warning?The message was sent from an email address in your Gmail contacts, but the content and/or other attributes of the message look suspicious..."
The from address was definitely not in my Gmail contacts. The message came from another email address and was pulled in by POP3.
I suspect there is a bug in the handling of phishing email from POP3 accounts that causes it to be treated as coming from a Gmail contact.