EDIT from below: "If I remember correctly, if you are the recipient of an email from "yourself" Google automatically puts it in the sent items label as well."
People are reporting these messages are in their sent mail folder!
Now it's possible that gmail makes it looks like it was "sent mail" if it receives a message "from you" even if you didn't send it, but it's also possible that google has been hacked in a big way.
And some of the people reporting this are knowledgeable people who know what spoofing is, and who have good unique passwords and two-factor enabled.
"If I remember correctly, if you are the recipient of an email from "yourself" Google automatically puts it in the sent items label as well."
Google threw it in my Sent Items despite me not sending it from gmail itself.