Have the player leave his bag away from the car or be suspect.
Again, players could go swap out for illegal discs now (how often does that happen?), and golfers visit their cars, the pro shop, the halfway house, etc. without this being an issue.
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The comparison to illegal discs is a red herring.
First, because an illegal disc may be spotted if thrown; a replacement for a lost disc won't be.
Second, there's little strategic advantage now to going to throwing an illegal disc. It's not going to make a huge difference in your score. There's no motivation to go the car mid-round and sneak one into your bag.
If the bag limit is effective in affecting scores, then there's a real benefit to sneaking in a replacement for the driver you threw in the pond, or even a strategic change in the lineup. Hence, temptation to do so.
For years I played tournaments with a 10-disc bag, and I made mid-round changes often, to my benefit, or at least I hoped so. Except, of course, I didn't have to sneak the discs. If it were a rule I wouldn't......but I know of players who would.
So, yeah, we might tell players to leave their bag with the group, or a group member might accompany a player to his car. Do we really want to go there?
It's admittedly a minor issue, but something to bear in mind with a potential rule. It's not the
reason I would object to a PDGA-wide rule, but it's something to consider and deal with. My objection is that it would be the imposition of an unnecessary rule, unpopular with the players, with enforcement issues, some annoying side effects, and
no benefit to the game. It might require more skill, or at least a different skill, but wouldn't make the game better.