This was interesting ..https://www.theregister.co.uk/2018/04/23/spammers_slipping_spoofed_gmails_sent_folder
Google has confirmed spammers can not only send out spoofed emails that appear to have been sent by Gmail users, but said messages also appear in those users' sent mail folders.
Punters have been noticing and reporting the problem for a few days on Google's Gmail help forum. In each case, users said that messages they never wrote were showing up in their folders.
This looks scary at first sight but thankfully whilst it's annoying it should not affect anyone, this makes a welcome change.