the person who found it was either looking for it, or just stumbled upon it - either way, they were obviously motivated my financial gain, because they know that exposes (that should have an accent on the last e) in the paper mean money for the person who provides the information.
personally, i would have handed it in to the police.
our security is almost always never secure enough - FFJ found ways to get batman and superman up and around secure buildings, and also to throw purple dust dombs at the PM - even though there were no leaked reports in those cases, they managed to get in and do what they thought thay had to.
i daresay that there is a "plan b" and no-one would be stupid enough to put all the plans into one report (oh please) so they'd go with "plan b" and keep an extra eye out.
there are people within the government who quite happily leak information out, some because they feel some sort of vindication, maybe; others again, for financial gain.
responsible members of the public would have handed it in to their nearest police station, rather than their nearest newspaper office. but if it was purposefully leaked, then the government needs to get a plumber in.