This'll probably sound INCREDIBLY dumb, but:...

How exactly does Souther work out that Tate's the leak just because he's told that Tate asked for the checks & bills to be left on his desk for authorisation? I mean, there was nothing dodgy about Tate doing that - if he lacked the proper authority then the clerk/junior officer would've immediately reported it to a superior; and since Tate had no previous inkling as to where Deloris was, much less that the convent donation was among the papers, the fact that he asked for them could've only aroused suspicion if for other reasons he'd already been suspected. Somehow Souther just works it out as soon as he learns Tate's doing the accounts, and starts running when he sees him on the phone - without even hearing who he's on the phone to. I don't know about America, but in an English court such flimsy 'evidence' would be thrown out faster than light. Please explain.