oke fair enough:P so i just walk over every briefcase dropped by a dead enemy?
Walk over every dead enemy if you can. Possibly a bug, but I've seen a few that dropped no visible cases but still had research when I passed over them. AFAIK it also seems that difficulty affects what research you can actually get, going into my second new game plus I bumped up the difficulty to Ghost and started getting research items I'd been unable to find before.