Originally Posted by
Some person
I think the risk is that someone wishes to buy a ticket on board (is this permitted on Thalys?) and so the conductor assigns your seat to that person, since no one claimed it within 15 minutes after departure from Paris. It's a lot riskier on trains without mandatory seat reservations as someone without a reservation could take the seat.
But it's a very low risk, how many people ever buy tickets on board, and how likely is it that the train is so full that *your* seat is the only available one to be assigned to a last minute purchaser?