And maybe he wanted to return the computer because he already purged the information that he wanted to remain a secret, and he figured that a sanitized computer would be less suspicious than no computer at all?
From what we know the man knew the victims name, he knew of his PC and Briefcase, and he knew not only that they had been found -- but where to gain access to them.
Assuming some innocence to start with like his ARMY Buddy called him on his cell phone as the bridge went down in a panic to save the computer and documents -- Tossing them out of the car window (Or maybe he was in a convertable -- I hardly think that person X traveling X miles up the highway would have had handy the equipment necessary to monitor local police chatter as to said items being recovered by the river. The idea that the items didn't go straight to the trooper's evidence room and to the mayor instead is very odd in itself
Add to that this Person X strolls up and is able to not only make the specific request but gets the goods as well.
NOW ASK YOURSELF WHAT ARE THE ODDS . . .
Timing a tugboat to knock down a bridge to get a guy who crosses at a certain time is a bit hard for me to belive.
Or checked out information, hard drives, etc. and then copied it.