I think after they sacked Ben Collins, they began rotating multiple Stigs. At least initially. Multiple Stigs increases the potential for a leak, but it also gives TG a stronger hand for dispelling rumours that any given driver might be Stig #3.
"No, it can't be Anthony Davidson, because we filmed that episode on 22 Sep, and Davidson was driving LMP1 at Austin, Texas that weekend."
In the first few episodes after they un-Stigged Ben, I carefully scrutinised the new Stig's identifying features, and came away convinced they were using different drivers. I also believe one of them was Davidson. Or maybe McNish. Or maybe a munchkin.
Boris Becker videoed a conversation between he and The Stig when he was TG's star in their reasonably priced car (which was the second episode with The Stig #3). Becker posted the video on his website, audio and all, but Andy Wilman found out before I did and threatened him with excommunication from the JC fan club if he didn't remove it. So he did, unfortunately before I heard of it.
But those who did see it remarked that it definitely was a man's voice, which put to rest the rumour it might have been Sabine Schmitz. And some opined he sounded English or Scottish. The specificity of 'Scottish' gives it a ring of authenticity, as I doubt most of Top Gear's non-UK fan base could tell a Yorkshire accent from Yorkshire pudding. Much less English from Scottish from Ozzie. One said the accent sounded like Jackie Stewart, without further noting that Stewart is quite Scottish, which I thought quite telling. But he was quite short (Jezza even remarked in his opening monologue the new Stig was even shorter than the Hamster) and both Alan McNish and Anthony Davidson are 1.65m (5' 5") tall.
But that was four seasons ago. They could have created an army of cloned Stigs since then, like Star Wars storm troopers.