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

This question is born out of curiosity, I am not a potentional criminal…, one would start to think such, since I asked some crime related questions lately :-)
I am a crime show enthusiast.

So, say you are about to do something ‘naughty’ and you need an accomplish that is totally unknown to you, how would you contact such person, without leaving trails for the police to discover?
Is it really impossible or are there (obvious) ways that I simply don’t see?