What does it matter? If you have evidence of a drug deal going down but have no idea who it is or where they will be, do they need to be worried about it?
Also, if it’s really needed that the message not be read despite it’s source and destination not being known, application-level encryption can be used. Basically the message is encrypted so that the people in the middle can’t read it and only the writer and intended recipient know how to decode it, despite that they’ve never met each other.