Abraham Lincoln once said that only three things are certain in life: death, taxes, and idiots mis-attributing quotes on the internet. That second one is represented by The Witcher 3's Hearts of Stone expansion, which sends the Taxman after players that may have used exploits to make a lot of money very quickly.
YouTuber WhatsMyGame discovered the easter egg, and recorded it in the following video, but I'll give you a quick precis first. At some point during Hearts of Stone, the Taxman will appear to confront Geralt over reports that he's made "a lot of money very quickly". He then accuses our hero of making use of a White Orchard exploit to harvest tons of rawhide (since fixed by a patch in hilarious fashion), or using a similar exploit to harvest pearls. He also accuses Geralt of walking into people's homes and nicking stuff. Which of course no self-respecting Witcher would ever do.
It's not clear if he'll appear if you haven't done any of these things (I presume so; how would CD Projekt be able to tell?), but you can always protest your innocence—and you'll even be rewarded for it with a lovely gift. A commenter under the video says they admitted guilt on all charges, and were charged 1000 crowns.
I just love the way CD Projekt handles stuff like this.
If you're wondering whether to pick up Hearts of Stone, you might want to peruse our review. (Thanks, Kotaku.)