There are certain times when it is obviously not the regular Stig. Random stunts like the Winter Sports special snowmobile jump- stuff that goes outside the skillset of driving really fast around a track. Then other times like the Renault F1 lap of the TG test track- I would speculate that it was a team driver. The mystery of the "real" Stig's identity has never been that big of a deal to me. If it isn't the guy they're talking about now, it's somebody else very much like him, a driver from a second-tier racing series with solid driving skills but with low enough star power that they'd consent to being mostly anonymous.