If I'm not mistaken, paper trail is proof of innocence.
knowing that the property involved in a financial transaction represents the proceeds of some form of unlawful activity, conducts or attempts to conduct such a financial transaction which in fact involves the proceeds of specified unlawful activity
So if someone writes you a check funded by drug sales, then you have to worry about the reason for the check. Two of the reasons that will get you in trouble if the transaction is
(i) with the intent to promote the carrying on of specified unlawful activity; or (ii) with intent to engage in conduct constituting a violation of section 7201 or 7206 of the Internal Revenue Code of 1986;
Another reason would be
knowing that the transaction is designed in whole or in part— (i) to conceal or disguise the nature, the location, the source, the ownership, or the control of the proceeds of specified unlawful activity; or (ii) to avoid a transaction reporting requirement under State or Federal law
A "paper trail" if proof of nothing, it just makes it easier to get caught.