The implications may be that the FSB provided specific signatures for them to look for, they came back when they popped up on a machine located at this contractors house, then further assessments were performed. In context that’s not far fetched at all.
How do you think FSB "came back" to the machine of the NSA malware developer who's in the USA? I think that's exactly what is not plausible. He surely isn't going to open a trojan named isthatyou.jpg.exe in the e-mail sent by them to him. He actually made such stuff (trojans or something) himself as he let Kaspersky's software automatically collect the sample of his "work in progress." Now the unnamed government sources "leak" this as a case of apparent "Russian hackers" whereas the only known hackers in the story are the NSA and the Israel's hackers who hacked the office computers of Kaspersky. Kaspersky's software just did what other antivirus software does too.
