How would one contact an unrelated, unknown person without leaving any trails?

Leave a cut and paste letter at the person’s house.

@redfeather He’d want to wear a good disguise so he isn’t recognized, or having a messenger drop it off.

[Lurking to get tips for the next time I’m at Starbucks]

Good point. Get an accomplice to drop off this note at the accomplice’s. Or, mail it from out of town.

There is a wonderful true 18th century spy story that I read about one time. Unfortunately, I can’t give you a reference. Two spies were suspected of exchanging information, but there was no apparent contact between them. The closest they came to each other was that they ate at the same restaurant at about the same time. They did not sit near each other or ever came close to one another.

The secret turned out to be that the two of them wore the same type of hat. They simply exchanged hats, which contained secret messages.

Please clarify, @rebbel: you’ve already enlisted the accomplice, and now you just have to make actual contact? or you have to start cold, getting the prospective accomplice to agree to work with you? Or are you going back even before that, to the initial step of finding (identifying, discovering) a person who could become your accomplice?

Like, say, out of the blue I decided to commit a fraudulent act in order to make a lot of money. I had a chosen victim and a plan, but I needed someone else’s help and had no one in mind. Are you asking how I would go about finding someone to help me commit this crime, without leaving any traces of our connection?

Exactly how you worded it in your second alinea thank you for writing my reply to you (only much better) :-).

It’s atrocious to my sensibilities that some people or organizations would wish to make anonymous communications impossible. It used to be very easy. There did not use to be any caller ID, and little ability to trace short phone calls even when the police were watching.

All you need though is an unwatched mail box, and anonymous paper and writing, and to not leave physical evidence such as body cells, oils, prints, hair on the letter nor envelope.

Anonymous Internet use could also be used.

@rebbel, my second paragraph?

Telepathy.

Thanks to all for your answers, it seems to me that it would be extremely difficult.
@Jeruba Yeah, paragraph it is, i thought alinea (which is Dutch) was also used in English…