The general rule, he says, is that if investigators see something of interest beyond the scope of the search warrant when they're looking at electronic files, they may need to go back to a judge and seek a new warrant.
"Consideration should be given to obtaining information from other sources or through the use of a subpoena, unless such efforts could compromise the criminal investigation or prosecution, or could result in the obstruction or destruction of evidence, or would otherwise be ineffective," the rules say.