Probably just stirred up yeast. If you sanitized your thief properly, it is EXTREMELY unlikely that anything was introduced. It's harder to infect a batch than you may think. I'm a bit of a curious guy myself, so much so that I'll take my bung out every few days just to take a whiff, and haven't had any infections yet. Hell, search up "open fermentation" on Youtube, and there's some pretty crazy ****. Has there been any significant fluctuation in temperature? It's unlikely, but even this could increase airlock activity.