How did a private message disappear?

[viewing ownly]

Today I received a short reply from someone new to interacting with, and later on in the day, his message to me was erased. How is a private message removed as if it wasn't sent in the first place?

I thought "left the conversation" indicated that the person had no desire for further communication, not an entire removal of their message by them or perhaps the moderator. 

[RadioRob]

Moderators cannot do this. Admins technically can do it from the database but it’s a pain in the neck. Unless a PM is reported to us for some reason, we don’t see/care about private messages. Most likely the person who sent it in the first place deleted their message.