A player returning without permission means his team has more than the permitted number of players; refer back to an England world cup match where we had 16 on the pitch for a short spell. I see no other sanction listed, it also puts responsibility on the opposition to count numbers.
3.2 Team with more than the permitted number of players
Objection: at any time before or during a match a team may make an objection to the referee about the number of players in their opponents’ team. As soon as the referee knows that a team has too many players, the referee must order the captain of that team to reduce the number appropriately. The score at the time of the objection remains unaltered.
Sanction: Penalty at the place where the match would restart.
3.2 Team with more than the permitted number of players
Objection: at any time before or during a match a team may make an objection to the referee about the number of players in their opponents’ team. As soon as the referee knows that a team has too many players, the referee must order the captain of that team to reduce the number appropriately. The score at the time of the objection remains unaltered.
Sanction: Penalty at the place where the match would restart.