I also happen to believe that if the reports are correct, this laptop was voluntarily surrendered. When police are given consent to search (either by the target of an investigation or by a third party with control over a premises) they do not need a warrant, and they do not have to be searching for specific items at all.
I tend to agree with you on the law, but agree with offduty on the need for a warrant to cross all the “Is” and dot all the “Ts.”
I think the law is clear that tangential searches are allowed under the plain view doctrine, but I could also see the contra argument where a motion to suppress may be granted if you go well beyond the scope of the warrant.
Rather than get unlucky with an Obama District Judge, an Obama Appellate Court and a crap shoot with SCOTUS (depending on who wins the election), getting a warrant would likely be the proper way to go.