This happened to me dealing a cash game years and years ago. I sit down, there is a seat with chips where you assume they're in the washroom or smoking etc...
Few hands later, a player sits there and I deal him in. He plays 3 hands, losing money in the first 2, and winning the 3rd pot.
Then a guy (Bobby) shows up yelling at this person to get out of his seat. The whole table starts laughing cuz this is Bobby's seat. That other guy mistakenly sat at the wrong table. He was in the same seat one table over.
Personally I was upset with the other 9 players at the table that didn't say anything when this wrong guy sat there and started playing. Like how am I supposed to know who's seat that is.
We ended up checking surveillance and making sure "wrong guy" paid Bobby back for any amount he had lost him. The people that had won $ in the first 2 pots got to keep their winnings. Whatever "wrong guy" won in the last pot was given back, we basically voided the last hand.
I was just a dealer and did what I was told, and this is an odd situation where there is not written rule and the floor person needs to make a call which he deems fair.
For the tournament situation, since it was only 1 hand, I would probably void that hand, have all chips go back to the players that contributed and deal that hand again with the puck in the same spot, with players in their correct seats.
Not sure where I stand on whether or not a penalty is needed for the player that sat in the wrong spot. It most likely was unintentional and an honest mistake, but then again players make mistakes all the time in tournaments and "intend" to do something else, and have to pay the price. (ex: string betting, silently throwing out an oversize chip and meaning to raise, etc)