They did take possession of the computers, but were searching foror evidence that
had been accessed. Was it common knowledge in 1996 that pedophiles would 'groom' children by pretending to be children themselves? In fact, it's the very thing that tricks the kids into giving information about themselves. Unless LE was specifically looking for it, I doubt it would have rung alarm bells with them if Burke had an 'online' friend.
Just from what I have read about BR, he was a bit immature for his age. In 1996 I don't think kids that age were as computer-savvy, nor did they usually have their own laptops or computers. I have a hard time envisioning him having cyber-friends. He was said to like video games, Nintendo and such, and that seems more likely. I am sure LE searched the computers for emails and chat rooms as well.