I was a little confused by that, too. At first, Rodie looked at the Major as if to say "he's full of it". Then the Major says, " I happen to know a detective spoke with you 18 years ago." After the call, there's an obvious edit, and we only hear Rodie say it was all over the papers. He didn't appear embarrassed to me, only PO'd.
Regarding collection of a surreptitious DNA sample, location might have presented a problem. Besides, they were able to learn a lot about the ex's attitude through the exchange, and I think they really wanted to see him in person and question him. IF DNA cleared the ex, then they keep looking.