I found another case where this invisible decryption occurs. I received a
message with some encrypted attachments. I clicked the decrypt button in
Outlook, and GPGOl prompted me to ask if it should decrypt and save the
attachments. When I clicked 'No,' it decrypted the message on disk (I could see
it change in the preview pane in the background) as well as in the message
window. I confirmed that viewing this message via OWA that it was in fact
decrypted on disk.