Begs the question, has anyone ever booked a player for leaving the FOP without permission? Entering FOP; yes, I could see that happening more often, but the only time I can think of a player going AWOL, is during a goal celebration (in which case half the team would need to see yellow)
Last season, player left to adjust equipment without telling me. He jogged back on as the ball was going into his half (second offence), I played on, corner kick. Stopped play there and cautioned him. He was surprised, I said if I was being strict he'd be off for the two offences. I made a topic about it on here last season.
And this season, a player went off after scoring a goal because he was 'winded'. This was a game that was heavy with dissent from his team. Everyone had got back to restart via kick off except this guy who went off to his bench. So I went over cautioned him, warned him not to come back on without my say so and restarted without him.
Btw, I'm pretty sure leaving the fop during a goal celebration is already in the LotG:
Leaving the field of play to celebrate a goal is not a cautionable offence but players should return as soon as possible.