What should the penalty be for a player leaving the playing venue?

FIDE Laws of Chess article 11.2 says:

11.2 The ‘playing venue’ is defined as the ‘playing area’, rest rooms, toilets, refreshment area, area set aside for smoking and other places
as designated by the arbiter.

The playing area is defined as the place where the games of a
competition are played.

Only with the permission of the arbiter can

a. a player leave the playing venue

b. the player having the move be allowed to leave the playing area

At a congress recently the following happened.

After the first 6 moves, which I played very slowly as I I had never seen the opening before, my opponent played his move and got up. I thought for 5 or 10 minutes, played my move and noticed that my opponent hadn’t come back. I got up, had a look around the playing area and couldn’t see him. I checked the toilets. They were empty. He wasn’t there.

I went to see the arbiter. “I’ve lost my opponent” I said and explained the situation. “There are some toilets at the very far end of the building” she said. “He’s probably gone there”.

It didn’t sound likely to me. I waited just inside the entrance to the playing area and 5 or 10 minutes later my opponent appeared. There had been insufficient time between rounds for him to get some food and so, rather than go immediately before the round started and lose 20 minutes or so (30 minute default time) he waited until part way through the game when the position was complicated and he would lose much less time.

Now that my opponent admitted leaving the playing venue during the game without the arbiter’s permission I asked what she would do about it. “Well, he’s ever such a nice chap. He’s a charity worker, you know. So, of course he wouldn’t cheat. He shouldn’t really have done it but no harm done”

As an arbiter myself I think the explanation the arbiter gave in this case is a good one to justify not throwing my opponent out of the tournament altogether. It is not a good one to justify not defaulting him. And certainly not a good one for letting him off completely.

I’ve never had to deal with this when I have been the arbiter but for a “cheating related” violation like this I would automatically default the player and then consider whether I believed anything more untoward had happened before deciding whether to let him continue in the tournament or not.

Have any other arbiters had this situation before? If so what did you do? If not what would you do? There is no one prescribed penalty in the FIDE laws of chess as far as I know nor even specific guidance (please correct me if I’m wrong) and so it is left to the discretion of the arbiter.