There's a quite old trick, a couple of months old already, I'm not sure if this is the one you are asking about.
The guys post here and there "by mistake" the private key of ETH wallets. The wallets have a good number of tokens and no ETH. As soon as you want to steal from them and you send ETH to have gas, a smart contract sends away that ETH, leaving the "public" wallet again without ETH.
Quite an elaborate scam...
If those scammers dedicated as much time to something constructive they could make a lot of money legally but instead they prefer to use their knowledge to scam, what you are describing is something that I never heard but thanks for letting us know, they make it seem as if they made a mistake when everything was planned beforehand and they want to take advantage of someone trying to take advantage over them.