Can a player be cautioned after the match has ended?

Answer 1

The referee is allowed to caution or send off a player or substitute after a match has ended. In most leagues, the resulting consequences (i.e. sitting out future matches) will be the same.

Even if a card is not shown, say a referee is approached in the parking lot, the behavior will still be included the game report and the consequences would still be the same. Showing the card is merely a method of communicating. The referee's failure to do so does not absolve the player from the consequences of their actions.