I think you're over-reacting just a bit. I'm not saying I wouldn't have felt the same way, but ...
Consider their side of things -- they have discovered accounting irregularities. Here too was a "bond of trust" that they discovered was being abused. I think they have a right to be a little paranoid. They are probably treating everybody the same way -- they need to determine how far up or down the problem goes. Accounting and IT tend to be "black boxes" in most companies, a group of people who perform their magic that nobody else can understand, and they probably need an independent third party to fully scope the damage. They aren't suspicious of you, they're suspicious of everybody -- really, they're suspicious of themselves and their ability to select trustworthy employees. They need to reassure themselves that this accountant was a fluke, and not a pattern of employee misconduct or a systematic failure of the hiring process.
Unfortunately, your honor has been broached, and it was not your fault -- it was broached by another person who failed to live up to his own ethical obligations. It's a very unfortunately situation to be in -- ask anybody who works at Penn State.
I had an ending, but I lost it.