The nation's more than 10,000 voting jurisdictions buy their computer equipment from privately owned companies. Sometimes the companies do this customizing themselves. It's going to be virtually undetectable if it's done correctly. But these tests are often woefully inadequate. You effectively have to trust the entire staff of the corporation that is producing your software.
Every single member has to be trusted.
Florida twelve years ago, when Democrat Buddy MacKay lost his Senate bid to Republican Connie Mack by a razor-thin margin after a recount. Discouraged, MacKay gave up. Now that I realize the level of sophistication, I realize how easy it would be for somebody to, in effect, rewire that program for a brief period of time and then have it straighten itself out. I don't know how anybody could prove that. You'd have to concentrate on a few states and counties and precincts that you'd need.